On the face of it, there is a tension in adhering both to the idea that there are facts about what it's rational for people to do and to the idea that natural or scientific facts are all the facts there are. The aim of this post is just to try to make clear why this should be so, and hopefully to get feedback on what people think of the tension.

In short

To a first approximation, a belief is rational just in case you ought to hold it; an action rational just in case you ought to take it. A person is rational to the extent that she believes and does what she ought to. Being rational, it is fair to say, is a normative or prescriptive property, as opposed to a merely descriptive one. Natural science, on the other hand, is concerned merely with descriptive properties of things -what they weigh, how they are composed, how they move, and so on. On the face of it, being rational is not the sort of property about which we can theorize scientifically (that is, in the vocabulary of the natural sciences). To put the point another way, rationality concerns what a thing (agent) ought to do, natural science concerns only what it is and will do, and one cannot deduce 'ought' from 'is'.

At greater length

There are at least two is/ought problems, or maybe two ways of thinking about the is/ought problem. The first problem (or way of thinking about the one problem) is posed from a subjective point of view. I am aware that things are a certain way, and that I am disposed to take some course of action, but neither of these things implies that I ought to take any course of action -neither, that is, implies that taking a given course of action would in any sense be right. How do I justify the thought that any given action is the one I ought to take? Or, taking the thought one step further, how, attending only to my own thoughts, do I differentiate merely being inclined to do something from being bound by some kind of rule or principle or norm, to do something?

This is an interesting question -one which gets to the very core of the concept of being justified, and hence of being rational (rational beliefs being justified beliefs). But it isn't the problem of interest here.

The second problem, the problem of interest, is evident from a purely objective, scientific point of view. Consider a lowly rock. By empirical investigation, we can learn its mass, its density, its mineralogical composition, and any number of other properties. Now, left to their own devices, rocks don't do much of anything, comparatively speaking, so it isn't surprising that we don't expect there to be anything it ought to do. In any case, natural science does not imply there is anything it ought to do, I think most will agree.

Consider then a virus particle - a complex of RNA and ancillary molecules. Natural science can tell us how it wiil behave in various circumstances -whether and how it will replicate itself, and so on- but once again surely there is nothing in biochemistry, genetics or other science which implies there is anything our very particle ought to do. It's true that we may think of it as having the goal to replicate itself, and consider it to have made a mistake if it replicates itself inaccurately, but these conceptions do not issue from science. Any sense in which it ought to do something, or is wrong or mistaken in acting in a given way, is surely purely metaphorical (no?).

How about a bacterium? It's orders of magnitude more complicated, but I don't see that matters are any different as regards what it ought to do. Science has nothing to tell us about what if anything is important to a bacterium, as distinct from what it will tend to do.

Moving up the evolutionary ladder, does the introduction of nervous systems make any difference? What do we think about, say, nematodes or even horseshoe crabs? The feedback mechanisms underlying the self-regulatory processes in such animals may be leaps and bounds more sophisticated than in their non-neural forebears, but it's far from clear how such increasing complexity could introduce goals.

To cut to the chase, how can matters be any different with the members of Homo sapiens ? Looked at from a properly scientific point of view, is there any scope for the attribution of purposes or goals or the appraisal of our behaviour in any sense as right or wrong? I submit that a mere increase in complexity -even if by many orders of magnitude- does not turn the trick. To be clear, I'm not claiming there are no such facts -far from it- just that these facts cannot be articulated in the language of purely natural science.