Given that it is possible to think rationally, it must be possible also to think irrationally. An analogy with the concept of biological function may usefully illustrate why this is the case. If some part of the body has a function--so, for example, if the heart has the function of pumping blood--it must be possible for it to malfunction, for otherwise the ascription to the heart of the function of pumping blood would not have any normative force. Similarly, insofar as one aims to think in accordance with the norms of thought, it must be possible for one not to think in accordance with those norms, and hence to think irrationally.

Your question goes to the heart of the basis for moral judgments and their justification. If moral judgments are supposed to reflect universal standards that are binding on all possible rational beings--Kant, for example, seems to conceive of ethics this way--then it would not be possible for a rational species to evolve that would not share the same moral judgments as all other rational species. If, however, one thinks that moral judgments reflect certain norms that are internal to a culture, and/or that reflect the ways in which members of that culture negotiate their relations with one another, then moral judgments might well vary with the nature of the species in question. (One can, for example, imagine a culture in which it was morally wrong ever to manifest any signs of pain or distress, for example.) The deep question here, it seems to me, is whether morality should be seen as applying to all beings of certain types, or whether it should instead be seen as a very particular, species-specific or...

Philosophers distinguish different types of rationality. The ability to judge whether means will achieve ends is generally called 'instrumental rationality'. Epistemic rationality consists in proportioning one's beliefs to the relevant evidence--although it's a nice and subtle question just what counts as 'relevant', and seems to me in fact to call for the exercise of epistemic rationality. A third type of rationality, related to the first, since it concerns actions, is practical rationality, or practical reason: the exercise of reason in forming one's intentions, or determining what one should do in a given situation. There may well be other types of rationality as well, but to my mind at least, the three types that I have identified continue to receive the most attention from philosophers. One issue that continues to be engaged concerns the relation between instrumental and practical rationality: some philosophers have claimed that practical rationality just is instrumental rationality; other...

This is a fascinating nest of issues!! It has been claimed that reasons are fundamentally different from causes, but it has also been claimed that reasons are causes--maybe a different kind of cause from the cause that makes it the case that putting a weight on a balance moves its arm downwards, but maybe not. (Even if reason is a indeed a cause of one's choice, it may not be the case that acting for reasons is therefore involuntary.) Regardless of whether reasons are or aren't causes, however, the question of whether reason alone can motivate action (or choice or decision), is a distinct, albeit related, matter. If reasons were causes, then it would seem that the mere recognition of a reason would be sufficient to move an agent to action. But of course it is often the case that one recognizes reasons and nevertheless does not act on them. Even knowing that smoking is bad for one's health, one may nevertheless continue to smoke. So, one might conclude, reasons aren't causes. However, perhaps...

On certain conceptions of free will, freedom is bound up with rationality. On other conceptions of free will, however, freedom consists in a capacity to be a first cause of one's choices or actions, and so on such a conception, freedom seems to float free of rationality. Indeed, on such accounts, to be determined by reason seems to curtail freedom. In the Essay Concerning Human Understanding , Locke raises a good question for accounts of free will that do not tie freedom and rationality closely together. "Is it worth the name of freedom to be at liberty to play the fool, and draw shame and misery upon a man's self? If to break loose from the conduct of reason, and to want that restraint of examination and judgment, which keeps us from choosing or doing the worse, be liberty , true liberty, mad men and fools are the only free men: But yet, I think, nobody would choose to be mad for the sake of such liberty , but he that is mad already."