• Stoicism uses the senses and the mind as a source of knowledge. For example, geology obviously requires sense data, while  mathematics can be done in a sensory-deprivation chamber. Other things (including probably many areas of geology and math) require both sense data and the mind. For stoics, those are the only two sources of knowledge: sense data and the mind. Stoicism builds up a rich and complex view of the world using those two basic tools. Among other things, stoicism observes that every collection of sense data requires a functioning mind. It also observes that we humans are rational and social by nature. We are other things too, but those two things are at the core of our nature. And they are facts, say the stoics: they are givens about us that we can abuse, repress, misuse, and ignore, but we cannot really change. They just are the essence of what we are.
• So what? The stoics think that from the FACT that we are rational and social by nature, we can derive the conclusion that we OUGHT to pursue our rationality above all and not ignore our social side either. They think that pursuing our rational side will automatically take care of our social side, because we cannot pursue our own rational self-interest without also furthering the interests of fellow humans. They think that is a fact about how humans operate. Whether they are right or not is an interesting question beyond the scope of this little letter. What I want to ask today is whether they are right to think that from a FACT about us, we can conclude what we OUGHT to do.
  
• First off, we are much more than merely rational social beings. We are also physical beings who engage in recreational activities, eat, collect things that we say we "own," and generally do all sorts of things that are plausibly considered tangential to rational and social concerns. Why can't I grant that I am by nature a rational and social being and nonetheless conclude that what I ought to do with my life is go live on a beach in Crete and pursue whatever passions happen to take me at any given moment? Why can't I notice that I get a big kick out of cow-tipping and just pursue that as a life goal? What is so important about our nature and developing it? Sure, the acorn is "by nature" such as to grow into a big oak tree, and so is arguably here for that, but we aren't acorns. If I were an acorn, I would have no choice but to "try" to be an oak. Acorns are like computer programs. They don't ask questions: they just do whatever they do once you've wound them up and let them go. We aren't like that. We can decide to do other things, things that don't change our nature, but are not essential to it.
  
• How can stoics think that from the supposed FACT about our NATURE (that we are rational), we OUGHT to devote all our energy to developing our rationality? Most philosophers think that the interface between statements that talk about how some thing in the world IS and statements which talk about how some thing in the world OUGHT to be or act are in need of careful treatment. Some philosophers even think that no OUGHT can ever be derived from any IS! Hence if I want to derive and OUGHT from an IS, I ought to tell you what an OUGHT is. Lawrence Becker (a modern stoic) defines it as: "to say that an agent ought to do or be X is to say that her doing or being X is advisable (but not necessarily required) in terms of some endeavor she has." That seems reasonable. Now, to apply the definition. Given that we are by NATURE rational and social, we cannot help but have at least some endeavors that significantly involve rationality. Given that any endeavor we undertake as humans involves many choices, and choices are nothing other than the engagement of the executive function in ourselves, and that executive function's best means to maximize our chances of success at any endeavor is to engage in some rational thinking, then it follows that given that we ARE rational by nature, we OUGHT to engage in rational thinking. Given that rational thinking is the most important part of any human endeavor of any size and scope, we ought in fact to be working to perfect our rationality to the best of our ability, as long as that does not interfere with any other higher-priority endeavors. While I have not (yet) argued that there are no other higher-priority endeavors, I have gone a long way toward explaining how the stoics think that starting from observations about facts about us, we can derive the conclusion that we ought to devote considerable energy to our rationality.