Aristotle on Change

Coming To Be: Change: Cause and Explanation

Physics 1.1 and Physics 1.7-8

To demonstrate that we have knowledge of something, we have to show that we understand the factors that explain it.

Aristotle calls these 'principles,' 'causes,' and 'elements' among other things.
To know what a cow is, we need to know many things, including: what is it made of? how does it grow? what is its purpose? how did it come to be? will it cease to be?

Physics 1.7-8

Nature is not random:

peaches don't grow on cherry trees,
cows aren't born to giant squids,
rocks do not originate in the tops of trees,
the air does not become gold,
cherry trees do not decay into brick walls or pizzas,
antelopes do not die and become icebergs or pine needles, etc.

Anything that comes to be F must come to be F from being not-F.

and not just any 'not-F,' but the one that is potentially F before it becomes F.
and something that is not-F in that way is a contrary of F.

contrary is nowhere near the same as a contradictory

Anything that ceases to be F must cease to be F and become not-F.

But not just any not-F.
E.g. when a pale thing ceases to be pale, it does not thereby become musical (which is indeed not-pale).
Rather, it becomes some other not-F related to paleness, such as dark, the contrary of pale. Many such contraries have no names: pale and dark, human bodies and corpses are among the few such things that have names.

All of this applies not just to simple things, such as a particular quality, a particular substance, etc.

It also applies to combinations, orders, and arrangements.
When they come to be or cease to be, they can only do so from or to certain other things.

A house, for instance, comes to be from what is potentially a house, and ceases to be into something that is not-house in a particular way (viz. a ruin, or recyclable materials, or landfill).

THINGS THAT COME TO BE OR CEASE TO BE DO SO FROM OR TO A CONTRARY OR AN INTERMEDIATE.

All comings to be or ceasings to be happen to a subject:

that subject may be one in number,
but it is not one in form.
It is not one in form, because one of its forms remains the same while another changes.
The one that changes comes to be its contrary or an intermediate thing between contraries, while the one that remains does not change.
Thus Aristotle speaks of the subject and the opposite in all cases of change: there is always a subject and an opposite.
Sometimes the opposite is merely privation (e.g. 'not-human') and has no common name.
All things that come to be are: they are a subject and an opposite.

Aristotle distinguishes between changes that involve coming to be F or ceasing to be F where F is a nonsubstance and where F is a substance.

Coming to be a substance or ceasing to be a substance is what Aristotle calls coming to be and ceasing to be 'without qualification'

We say that a human comes to be, not that food or an egg 'comes to be human' is A's idea.

Other comings to be and ceasings to be are all coming to be some (accidental) thing or ceasing to be some (accidental) thing: a human comes to be musical, for instance (we don't say that 'musicality' comes to be).

In cases of coming to be or ceasing to be without qualification (i.e. substantial coming to be), the substance does come to be from something or cease to be into something.

Some do so by changing their shape (a lump of bronze comes to be a statue by change of shape),
some by addition (eggs come to be animals by addition),
some by subtraction (carving stone involves removing what is not-statue from what will be the statue),
some by composition (houses are put together),
some by alteration (A gives no specific example of this).

A also distinguishes between coming to be essentially and coming to be coincidentally:

The musical human coincidentally comes to be a ping-pong player,

but the non-ping-pong player (a potential ping pong player) comes to be essentially a ping-pong player.

Note that both the musical human and the non-ping-pong player are the same person:

so every coming to be involves one essential coming to be and a possibly infinite number of coincidental comings to be.

True knowledge of a coming to be involves the essential coming to be, not the coincidental ones.

Also, the product of the change can be described coincidentally:

the acorn comes to be an oak essentially,
but it comes to be an object liable to lightning strikes coincidentally.

The dark thing remains the same thing, but comes to be the pale thing:

paleness comes to be from darkness (which is potentially pale) and the subject is the thing itself that is dark or pale.

Aristotle points out that in any instance of coming to be of a natural thing, there are 2 entities:

the subject of the change and the form that the subject comes to be.
The lump of bronze and the form of the statue, for instance.

In another way, there are three things:

there is the lump of bronze, the quality absence-of-form-of-the-statue, and the form of the statue,

which are the subject, the privation, and the form.

The form and the privation are contraries.

In the case of substances, even if there is no name for the substance, and even if it never exists separately, we can identify the subject of the change by analogy:

as the bronze is to a statue, so the subject is to the particular substance.
I'm trying to think of a good example of a substance with no name that never exists separately (from what?): can you help?

Aristotle improves on the theories of change of his predecessors.

They said that if something is, then it is, and if it is not, then it is not.
They concluded that, contrary to appearances, nothing can come to be, for that would involve it coming to be something from nothing, which cannot happen.
Aristotle's improvement is to add the concept of a subject underlying change and the idea that that subject has potentiality for change. Aristotle says that change is the actualizing of a potentiality of the subject. That actualization is the composition of the form of the thing that comes to be with the subject of change.
Another way to speak of change is to say that F comes to be F from what is not-F.

Only not-F things can come to be F.
But not just any 'not-F' will do: in order for a thing to come to be F, it must be the sort of not-F that is accurately described as 'potentially F.'
Another way to put that is to say that while dogs are 'not-statues,' they are 'not-statues' in a different way from the way in which a lump of bronze is a 'not-statue.'
To give it a name, the lump of bronze that is not a statue has associated with it the quality 'not-statue' as a privation, whereas the dog does not.
NOW, the question is, has Aristotle merely moved the question to another place: if the problem his predecessors had was that change cannot exist unless something comes to be from nothing, hasn't Aristotle merely moved the problem from a question of how a whole thing can come to be from nothing to a question of how a not-F (and an F) can come to be from nothing? NO, because his explanation posits that all things are not-_____ in an indefinite way (each one has an indefinite range of things that can fill in the blank of not-_____), and once one of those privations is actualized, the new F that it is has built into it a whole new range of not-____'s that it is potentially.

Physics 1.9

Chapter 9 is essentially concerned with arguments against Platonic conceptions of change.

Physics 2.1-3

Natural things are things such as animals, plants, chemical elements.

By 'natural' here, A means "having within itself a principle of motion and stability in place, in growth and decay, or in alteration."
The principle is part of what A calls the 'nature' of a thing.

Things that are non-natural include all artifacts, all things made by humans.

They are not natural because they lack such a principle within themselves.
A bed may change, but only insofar as it is acted on from outside of itself or insofar as it is constituted by some natural material (which is the artifact's matter, but not the artifact itself): it won't change as a result of some principle internal to bedness: rather it will change as a result of some principle internal to the material it is made of (wood, metal, etc.)
There are also some things which are the product of luck/chance: things such as a fire due to lightning.

But in those cases, the lightning is natural, and the tree it strikes is natural.
What is non-natural (although not at all contrary to nature) is the fact that that particular tree was struck by that particular lightning.

At times, an artifact may seem to be the same thing as its producer:

a doctor who cures herself.
The cured body is the artifact, and the doctor is the body, in a sense.
Thus the doctor has within herself the principle of alteration.
But that does not fit Aristotle's definition of 'natural,' because the doctor and the curing are not necessarily connected:

it is coincidental that the person being healed is the same person as the doctor.
The principle of change is within the cured person, but not in her in her own right.
The sick person is not cured qua doctor, and the doctor does not have herself qua doctor as the object of the medical art.
Rather the doctor coincidentally has herself as the object of her medical art.

The things that have a nature are the things that have the principle of (some of) their own change within them, and they are all substances.
Things that are in accordance with nature are both substances that have natures and the things that belong to such substances qua things with natures.

E.g. going upward belongs by to fire in its own right, and so going upward is in accordance with the nature of fire.

Aristotle has little patience for people who claim that there is no such thing as a nature.

He holds it to be as obvious as the brick wall that stands in front of you.
His reasoning is that denying that things have natures involves using something that is far less knowable in itself to deny something that is knowable in itself.

(He had better have more to say about it than that, but that will suffice for us for now. The argument that bricks exist because this one just bashed my skull is not in all likelihood such a bad argument, although it is not one amenable to formulation in words.) Perhaps what he has to say is that passage we analyzed in which he rejected the idea that things are just bundles of accidents as incoherent.

Aristotle considers the question of whether material elements are the true best candidates for substance and nature.

I.e. are we really just our chemical elements in some arrangement with the traits that result from them and their arrangement?
Aristotle agrees that that is in fact one way to speak of the nature of a thing.
There are good reasons for that: those things are everlasting and do not themselves change (in our earthly realm, aside from nuclear reactions).

But Aristotle still wants to say that the form and the account of the form is a better way to speak of the nature of a thing.

His argument is that what is only potentially flesh and bones just is not flesh and bones: certain of the attributes that differentiate a living animal from a corpse are so important that when we ask "what is this animal?" we should mention them first, and only mention the fact that the animal is made of flesh and bones later, or perhaps as a part of our account of the form of the animal.
The composite of matter and form is not itself a nature, but is in accordance with nature.

There is a difference when we talk of artifacts in these terms from when we talk of natural things.

A bed, for instance, does not produce further beds, and does not become another bed.

A wooden bed will catch fire or rot qua wooden, but not qua bed. Bedness does not account for its changes.
But a human produces further humans.
The bed decays into wood, metal, fabric (whatever it is made out of) and the things that wood, metal, fabric, etc. can become in accordance with their natures, not in accordance with the nature of a bed (beds have no nature: they aren't natural).

Why can't I claim that the bed becomes a sofa (just add pillows), a jumping platform, a seat, or a table in accordance with its beddiness? Isn't that a principle of change?
Because the bed does not itself change in that way as you or I or an earthworm change location, shape, etc. in accordance with our internal structure.

Another example which Aristotle brings up: medical science

Aristotle claims that medical science produces health via treatment

and medical science does not proceed toward medical science as its goal.

What if I claim that medical science not only produces health but also further medical science, just as humans produce not only artifacts but also further humans?

How would A counter that claim?

Perhaps he would say that just as the bed by itself cannot change itself, so medical science cannot change itself. It is an artifact, just like a bed, but its principle of change is not within itself. Rather, it is within humans. Does that work?

In the case of natural objects, say an acorn, it proceeds towards and from its nature, an oak tree. We say that the acorn is growing into an oak tree (which is a producer of acorns), but we say that medical science is a producer of health (which is different from medical science and does not produce medical science?).

Physics 2.2

The mathematician studies surfaces, solids, lengths, etc. and studies them in themselves, not qua qualities of natural objects (or even artificial objects). No, the mathematician separates them from what they are qualities of. Mathematics does not define its objects in terms of change, matter, etc. The definitions do not include any matter that is the subject for change in mathematics, but students of nature do study things that have matter that is the subject for change.
The student of nature needs to study both matter and form. As with crafts, the crafter must know the material as well as the form. With crafts, there is a user and a producer. The user knows the form (e.g. the pilot knows what the rudder must do), whereas the producer knows the material (what can be made to do that). In crafts, the two are separate in a way. In natural objects, the two are not separate.
The student of nature must know the matter that is matter relative to the form that the student is studying. Just as the doctor needs to know primarily about the matter of a person qua sinews, flesh, bone, etc. and not qua carbon, oxygen, much less quarks, neutrons, protons, etc.
The four causes: four ways to explain a thing.

Material: fire and earth and air and water cause bodies, assumptions cause conclusions, parts cause a whole
formal: the whole causes parts, the form of a body causes the matter to be a body, the conclusion causes the assumptions to be assumptions of it
final: the goal of a thing causes it to be
efficient: the seed causes the tree, the doctor causes health, the general causes defeat or victory, etc.

Final cause is the goal, not necessarily the beneficiary.
Fitness is the final cause of hard work, but hard work is the efficient cause of fitness.
A pilot can, by presence, cause safety of a ship, and by absence, cause shipwreck: in both cases, we say the pilot is the cause. Thus the same thing can cause contraries.
Causes can be arranged in a branching tree: a doctor causes health, but the doctor is a crafter, so a crafter causes health. The crafter is farther from health than the doctor is.
Causes can be coincidental: Polycleitus carved a statue, but Polycleitus qua sculptor is the efficient cause of the statue, while Polycleitus qua human being is only coincidentally the cause of the statue, and Polycleitus qua pale human is more remotely the cause of the statue.
The efficient cause can be spoken of as a potential cause or an actual cause: the housebuilder is a potential cause of a house before she builds it, but the actual cause while she is building.
Causes can be spoken of in particular or in general: for instance, parents generally are the cause of children generally, but your particular parents are the cause of you in particular.
We should seek the most precise causes, the ones closest to and non-coincidental to what they are causing. If we are seeking the cause of a particular, we should seek particular causes. If we are seeking causes of genera, we should seek generic causes.

Physics 2.4-6
2.4

Luck and chance are carefully defined by Aristotle.
He starts out with puzzles about them.
A scenario: Al goes to the market. Pat is at the market. Al had no knowledge that Pat would be there. Al meets Pat.
What caused the meeting?

1) Luck?
2) Pat's wanting to go to the market; Al's wanting to go to the market.

Those who say 2 identifies the causes are in general claiming that everything that seems to be caused by luck has fully explanatory causes that have nothing to do with luck.
People nonetheless continue to speak of luck as a cause. What is with that?

There are clearly things that do not seem to be the result of luck: apples don't just happen to grow on apple trees. There is an essential connection between apples and apple trees. In general, things that come about invariably or for the most part in the same way are not caused by luck. But there are things that come to be only occasionally or uniquely: those seem like candidates for being caused by luck. But in a way that is not the only sort of thing that Aristotle says can be caused by luck: things that come about for the most part can have particular instances that are caused by luck.
Of things that have a final cause, some are in accordance with a decision, others are not (the argument that things that have a final cause must have a designer behind them seems to work well here). Aristotle defines luck and chance: things that come about usually as the result of a final cause, but in some cases (the ones caused by luck) do not come about as the result of a final cause are caused by luck or chance. For instance, meeting someone you know you want to meet is usually the result of a decision that leads to action, but sometimes it is not. When there was no particular decision that led to a particular meeting with that particular person, we say that the particular meeting was caused by luck. Note that particular things are caused by luck. General things cannot be caused by luck. In the case of a meeting caused by luck, we say that it has a coincidental final cause (the goal of meeting that person). There can be an unlimited number of coincidental causes of any thing. The coincidental causes that could be caused by a final cause are instances of luck or chance.
'Fortunate' is a term applied to events caused by luck which are of no small significance.

Luck is a specific sort of chance. Chance is the more general term.
Luck is that type of chance which is found in things that are capable of being fortunate and capable of intentional action (i.e. agents, such as humans). Hence children, beasts, and inanimate things are not able to be lucky (or unlucky), although they can do things by chance.
Chance applies to all things that are incapable of intentional action: a horse can be saved by chance. It chanced to come out of the barn before the fire (but that can be seen as if it were intentional). A tripod can be saved by chance: it chanced to fall off the altar before lightning smote the altar (again, by a stretch, that could be seen as if it were intentional). Chance applies in cases where the event might have been caused by decision in the following sense: the horse could have been led out of the barn in order to be saved; the tripod could have been removed from the altar in order to be saved. Chance also applies to things that it is very difficult to conceive of at all in intentional terms (there chanced to be a meteor crash): that seems to contradict some of the things that A says, but it fits with other things he says.
Some events are contrary to nature: siamese twins, for example, are not 'normal' (which does not mean "normal" in any 'normative' moral or value sense: rather in the simpler sense that they do not happen invariably or for the most part). Those events are caused by chance according to A. Even though there being siamese twins in every particular case of siamese twins has a particular cause, and that cause is internal to the twins, nonetheless it is caused by chance. Thus a chance cause can be internal or external.
The paragraph at 198a7 is important and difficult. JL Ackrill in Aristotle the Philosopher, P 40, says that "Luck and chance, he is claiming, presuppose patterns of normal, regular, goal-directed action ('mind and nature'); and so it would be absurd to suggest that everything happens by luck and chance. We can have reason to say that some things happen as if they had been planned only because we take for granted that some things happen because they really have been planned; and we can pick out certain sequences as irregular and exceptional only against a background of sequences that are regular and to be expected." In other words, it seems as if Aristotle wants to say that chance and luck are happenings that are exceptions to regularities. He wants to say that regularities, by the very fact that they are regular, can be seen as having been caused for a reason, i.e. by an immediate final cause (not a coincidental one).

Physics 2.7-9
2.7

Here, Aristotle connects the efficient, formal, and final causes in a strong way: in the case of natural objects, they often amount to one thing. A human is the formal, efficient, and final cause of another human. Things that generate motion/change by being in motion are generally of this sort: the final, formal, and efficient causes are one.

This chapter considers the question of whether everything in nature happens by chance and necessity OR because it is directed towards an end.
IN ALL, it seems that we have two major possibilities for understanding Aristotle's final causes:
1) they are heuristic: they do not identify some actual thing that exists. Rather, they are descriptions of things that exist (material) that help us make sense of them. They are primarily epistemic entities. The material causes are sufficient to explain natural objects, but it is extremely useful nonetheless to think of natural things as being for something, because it furthers our understanding.
2) Aristotle thinks that final causes actually exist as independent explanatory factors: it takes more than just the proper material to cause a natural occurrence (a human, fire going up, etc.). It takes, in addition to the matter, a further explanatory factor, which is the final/formal cause. This is extremely hard for moderns to swallow, and that is because we have a very hard time figuring out a coherent notion of final causes, unless we postulate a designer-god-like entity, which is just what Aristotle seems to deny. There are certainly many very intelligent moderns who say that materialism is not sufficient to explain consciousness, for instance, but they do not seem to take a tack that is clearly Aristotelian.
#2 seems to be what Aristotle intended, because it best fits the textual evidence. But there are those who say he intended #1.
A modern formulation of the dilemma Aristotle is trying to get out of: if we put baking soda and vinegar together in certain circumstances, they will foam up. Putting those two materials together is sufficient for the foaming up. There is no particular reason to dream up some final cause for the event: the materials alone are sufficient, because they have certain chemical traits that inevitably lead to the foaming up, given appropriate ambient conditions. What if animals are simply a more complicated version of that phenomenon? take primordial soup like that of earth before life, and the material things that are in that soup are sufficient for life to happen is the idea. No need for a final cause!
Aristotle puts the dilemma in an even more pointed formulation that smacks of evolution: he describes the possibility that only those traits that happen to be conducive to survival actually do survive, while traits that do not make animals suitable for survival die out. No need for final causes there.
He rejects that possibility via an argument:
A dilemma argument:
Such things are either coincidental results OR they are for something.
They cannot be coincidental.
Thus they must be for something.
Why can't they be coincidental?
Because things that are coincidental are the result of chance or luck, whereas things that are natural happen always or for the most part, and things that happen always or for the most part cannot be the result of chance or luck.
In other words, nature is largely regular, chance cannot be regular, and so natural events cannot be by chance. They must be for something. This gives us some insight into what Aristotle means by 'final cause.' Namely, Aristotle does not want to say that there must be a designer behind natural things. Rather, he wants to say that material mechanical processes are not sufficient for living beings to occur. What is required in addition is what he calls the 'final cause,' a striving for being that occurs in addition to the material mechanical processes.
The analogy between nature and crafts shows that the same arrangement of sequenced events occurs in both, and that those sequenced events have the same basic structure in both cases. It is correct to speak of purpose in crafts, and so it is also right to speak of purpose in nature. But nature's purpose is not the intentional purpose of a human agent, rather it is a purpose without a conscious being that formulates the intention. It is simply a striving that inheres in substances (e.g. earth strives to go down, fire strives to go up).
Aristotle basically claims that where there is a regularity, it is often allowable to speak of a goal, even if there is no agent there capable of formulating purposes. Even if we do not believe that spiders are capable of rational decisions and there is no rational decision-maker that made spiders that way or continually direct them, nonetheless, we say that they build a web in order to catch a fly. Single-celled creatures wave their cilia in order to move towards food or away from threats. When we look for the scientific explanation of things, we do not exclusively look for non-purposive necessary regularities: there is something very informative about saying that web-weaving is for catching food. Saying that is simply a way of saying that this regularity generally exhibits the same relationship between events as clearly purposive action does (building a web is to getting food as bringing money and going to the grocery store is to getting food: it is activity for an end). This all makes the final cause seem like an epistemic entity (#1 above).
Deviations from such regularities are possible both in nature and in crafts: that explains why things in nature occur 'for the most part,' not invariably.
Some regularities, however, are purely matters of necessity and involve no purposiveness: Aristotle allows for that. Some such regularities are presupposed by goal-directed regularities: it is hard for me to find an example of this (which may mean it is not a coherent notion?). Others just come along with them: e.g., exercise is for health, but it is not for sweating. Sweating is coincidental to exercise, but a regular coincidence of exercise.
But what about the question, "What is the human function?" (or what is the canine function? etc.) It makes sense to ask "what are hands for?" but it seems to make little sense to ask "What is a human for?"
Aristotle clearly says the human function is to be as good a rational agent as possible. Aristotle also says it is to maintain the species. What is the species for? is the next question. Aristotle says that everything is striving to imitate his god, the eternal and unchanging activity of god: that is connected to being as good a rational agent as possible.

Here Aristotle introduces two notions of necessity:
conditional necessity: if we are going to have hamburgers, it is necessary to have chopped meat and buns and a grill.
unqualified necessity: given X, Y must follow.
BTW, Aristotle also recognizes a type of necessity that attaches to things that have happened or are happening: for example, it is necessary that I be at my desk typing right now in at least the sense that because I am actually here at my desk, I simply cannot be anywhere else right now. We'll call that de re necessity.
In nature and crafts, the necessity involved is conditional: if there is to be a wall, there must be materials to build it, but that does not make it the case that if there are materials, the wall must be.
Just so, if there is to be a cow, there must be flesh and bones, but flesh and bones do not necessitate the cow.
To make a wall, materials are necessary, but not sufficient. One must also understand what a wall is, and doing that involves knowing what it is for.
Just so, for there to be a cow, the flesh and bones (or food, or whatever the material components are) are necessary, but there must also be the form of cow, which involves what the cow is for (growth, locomotion, perpetuation of the species, and to be as godlike as the species cow can be).
Anything that has a goal has material as a conditional necessity: if it is to be, the material must be available.
The following may be unqualified necessity: given the proper ambient conditions, if you plant an acorn, it must grow. Generally speaking, put some matter with a privation/potential in the presence of the things conditionally necessary for the privation/potential to become actuality, and by an unqualified necessity, the potentiality/privation will become actuality.

de Generatione et Corruptione 1.1 and 3-5

Here Aristotle is concerned with what his predecessors had to say about coming to be and passing away.
When we die, does anything really cease to be? If we are just our material constituents, then the answer is no, for they are still there. Thus anyone who thinks that we just are our material constituents must call death a change, not a passing away, and an accidental change at that. Those who think that we are something in addition to our material constituents, however, may or may not think there is a passing away. They might think that the soul continues just as the matter does. In that case, death is just a change of circumstance for the soul, not a true ending.
Aristotle apparently thinks that when a thing comes to be, its form comes to be at the same time as the thing. For natural things, it takes a natural thing of the same species to create a new natural thing: it takes a human as the efficient cause to impart form to the matter. The new human is not identical to the father, because of the matter, which resists form. When a natural thing ceases to be, its form too ceases to be.

Perhaps the debate about whether forms are particular or generic for Aristotle has a place here: when I came to be, surely my species-form did not come to be. At most, we can say that an instance of my species-form came to be. But if I have a particular form, then my form could have come to be when I came to be and could pass away when I pass away.

In the case of primary substances, the primary beings, how can there be a coming to be and a passing away?
Aristotle thought that coming to be and passing away actually occur. The coming to be of a composite of form and matter was the true beginning of a thing, not simply an alteration. The dissolution of that composite was a true death, not just an alteration.
When a new substance comes to be (a child is born (well, perhaps conceived)), does something come to be from nothing? That would be a problem, since Aristotle rejects coming to be from nothing. Aristotle needs to explain how this works.
Aristotle's way of solving part of the puzzle is via his analysis of change (an underlying subject that underlies the move from a privation of a form to the presence of a form). The underlying subject must be in a state of potentiality relative to what it becomes: not just any old thing comes to be from any old thing.
Every coming to be is also a perishing. When the underlying matter receives a form, the form of what it was before perishes. When it loses its form, a new form takes the old form's place. Thus there is a continual perpetual coming to be and passing away.
But the coming to be of a this, a primary substance, is still a problem, for it is a being in a way that is independent of other beings. What underlies the change? The matter: it does not come to be. The form, however, comes to be (its source is another form of the same sort, but the new form is a separate form, which in a sense comes to be from nothing!).

de Partibus Animalium 1.1, 1.5
1.1

The discussion is abstract here. It concerns the proper procedure for the biologist: examine the species one by one or examine the things that are common to more than one species (sleep, growing, death, birth, etc.)?
The primary object of study should be the formal-final cause, for that is the key to the rest, just as a housebuilder needs the blueprints first.
Biologists should refer their accounts back to what is of necessity: the sort of necessity which the natural scientist pursues is conditional necessity (if this is to be, there must be that, and if that is to be, there must be those, and if those are to be....: first this, then that, then those, then these, then... until we get to the end for which a thing comes to be and is).
A problem arises, because it is difficult to provide demonstrations with things that are only conditionally necessary.
We should proceed by looking at how an animal is, and then say the reason why the animal is that way. In other words, for example, first we say what a human is, then we say that humans therefore have certain parts, since humans cannot be without those parts. We should produce explanations that are as close to that model as possible.
Following that, we should say how the parts come to be: the material explanation of the parts.
In other words, Aristotle wants us to look at both the function of the parts of animals (how they relate to the whole organism) as well as the way the parts themselves are constituted. The function is primary, however.
Saying that a hand has this shape and these properties is not sufficient: for it is conceivable that a wooden hand can have that shape and those properties, or a severed hand might have that shape and those properties. The only hand that is really a hand is connected to a whole and living organism. The severed hand is sufficient if all we want to know about is what hands are made of and what shape they have, but that is not what a hand is and does not sufficiently explain hands. We need to know what they are for and how they relate to the whole.
The nature of an animal is the animal's soul, and that is the primary proper object of study of the biologist. The soul is the efficient and formal and final cause of the animal.
There is a danger, however, in saying that the proper object of the biologist's study is the soul, for the soul is not just a thing separate from others. It is also the thing via which we understand everything, and thus the biologist's object of study might include everything. Aristotle rejects that idea, because understanding is found only in the human soul, and so when we study animals, we need not include everything.
Many items recur here that we saw in the Physics before:
The objects of the biologist's study, natural things, all exist for some end, as is apparent from the fact that not just any old thing produces just any old thing.
The full-blown adult animal is prior to the egg. The oak tree is prior to the acorn.
The sort of necessity biologists deal with is conditional necessity: food is necessary for growth, but growth will not necessarily result just because there is food. A seed is necessary for a tree, but a tree will not necessarily result just because there is a tree.

Natural objects fall into two sorts: the imperishable and the perishable. The imperishable are in the superlunary realm and are not as easily studied. The perishable include plants and animals, and there is ample opportunity to study them.
Aristotle clearly enjoys and perhaps feels defensive about investigating various animals and plants and encourages others not to disdain the study of them. Importantly, he emphasizes the need for observation in the field.