Rationality is the property of doing or believing what reason requires. There are two types of rationality:
- epistemic rationality is about what we believe, whether our beliefs are rational given the evidence we have available to us. For example, Tom believes his teacher is an adulterer because he read it written on the walls of the school bathroom, and because he doesn't like his teacher. Tom's belief is irrational, because it is based on an unreliable source of knowledge (the bathroom wall) and his own personal dislike
- instrumental rationality is about doing what is sensible given our goals. Tom wants to go to law school one day; but instead of studying he plays computer games. Tom's behavior is not rational, because it undermines the goal he has set for himself. On the other hand, if Tom wants to have a career as a janitor instead of going to law school, then playing computer games is not irrational for him.
Epistemic rationality is about conforming our beliefs to the evidence avaialble. Rationality and truth are not the same thing; if all the evidence supports a falsehood, then it is rational to believe it despite its falsehood. Likewise, something may be true, yet irrational to believe - considering the example above, Tom's belief that his teacher is an adulterer is irrational, since he bases his belief on unreliable evidence; it may well be that his teacher is in fact an adulterer, but while Tom's belief would then be true, it would still be irrational for Tom, since Tom lacks any good evidence for that belief.
Rationality and ethics
Both rationality and ethics are deeply connected, in that they are both systems which value events positively or negatively, and impose upon us obligations and prohibitions. Ethics labels actions as good or bad; rationality likewise labels beliefs as rational or irrational. Ethics says, Do this, but do not do that; rationality says, Believe this, but do not believe that.
Very often, ethics requires us to be rational. If I am a doctor, and I am going to prescribe a treatment to a patient, I must rationally believe the treatment is effective. Suppose I have a patient suffering from cancer; I read about a new anti-cancer drug in a reputable medical journal, so I believe it is effective, and prescribe it to my patient. I have done the right thing by my patient, ethically; even if it turns out the drug is ineffective or harmful, I discharged my ethical duty to my patient based by following the best information available to me. Suppose instead, I went to a psychic and asked what I should prescribe my patient for their cancer. The psychic responds, The spirits tell me, your patient has cancer because they did not eat enough cabbage!. So, I prescribe them a diet of cabbage as a treatment of cancer. Here, I have ethically wronged my patient, because I have prescribed a treatment, which while I believe it to be effective, my belief that it is effective is irrational.
Rationality and atheistic materialism
A purely materialistic worldview cannot adequately explain the objectivity of ethics, nor can it adequately explain the objectivity of rationality. By contrast, a theistic worldview can give a coherent explanation of the origin of rationality - God's mind is perfectly rational, and our minds are rational insofar as they operate like God's mind. The atheist cannot have an objective standard of rationality, so their rationality is reduced to nothing more than their own personal biases and prejudices.
Atheists attempt to cover over their lack of any rational foundations by pretending that what is rational is undisputed and obvious to everyone. However, just as there are genuine disputes concerning what is right and wrong, there are also genuine disputes concerning what is rational and irrational. Atheists have no way of resolving these disputes other than by appeal to their own personal whims or fancies, because they lack any objective standard against which those disputes can be measured. Religious believers, by contrast, have an objective standard for rationality, the perfectly rational nature of God.