“”Faith is believing what you know ain't so.
|— Mark Twain|
Faith is belief in God, gods or the doctrines of a religion; for example Christian beliefs are often referred to as "the faith". More broadly, and in informal/colloquial usage, faith is trust in an idea, or a belief that is unsupported by evidence or lacks proof. Faith is set apart from reason in that the latter uses deductive reasoning from evidence to inform beliefs, while the former relies solely on assertion to state beliefs are true. There are some arguments to suggest that reason and faith are, therefore, polar opposites.
 Different meanings of faith
The secular meaning of "faith" that is not centred around religious doctrine allows for multiple uses of it. Most broadly, if faith is taken to mean "without proof", many things are taken on faith - because strong, formal proof is really only applicable to mathematics. Things that are "taken on faith" have varying degrees of rationality associated with them, depending on how wild the inference actually is. For example:
- I have faith that the sun will appear tomorrow morning. (Based on past experience and very rational)
- I have faith in the legal system. (Based on a sometimes over-credulous faith in human nature, and a little less rational)
- I have faith in Jesus Christ our Savior. (Based on myth and totally irrational and without reason)
Sometimes "have a little faith" is essentially saying "don't be so pessimistic", which may or may not be justified depending on the situation. "Faith in oneself", for example, could be defined as having a rational trust in one's abilities, without unjustifiably assuming they are useless. The key point is justification: when we exhort someone to "have faith in yourself", we do it because we believe there is good evidence they can trust, not in despite of there being none.
"Blind faith" is a term that describes a faith-like assertion with completely insufficient evidence. Although "blind faith" is just really "faith", it is used to distinguish between religious faith, which requires only trust in the belief, and broader faith, such as trusting that inductive reasoning will turn out correct.
 Faith as a virtue
“”(...) blessed are they that have not seen, and yet have believed.",
The word "faith" carries many positive implications of trust and loyalty, but in normal life it's not taken to be an unbeatable virtue. Every (fully mature, mentally healthy) person has a limit to their trust and their loyalty. We warn children against untrustworthy people precisely because they don't know when it's appropriate to trust/have faith and when not to. We don't encourage them to belief that jumping out of trees wearing superman costumes is a good idea. Praising unquestioning faith - or unquestioning loyalty - in anything is objectively dangerous. Even in a traditionalist, hidebound society, normally the only entities who are upheld as deserving unquestioning loyalty are the highest authority figures and prestige institutions, not just ordinary people. And even in those cases it's not in any sense justifiable. The problem is that people who claim that faith is a virtue in itself always want to make exceptions for things that it's not okay to have faith in — a Christian fundy is unlikely to praise devotion to Osama bin Laden, for example.
 Faith quotations
- "Faith is powerful enough to immunize people against all appeals to pity, to forgiveness, to decent human feelings. It even immunizes them against fear, if they honestly believe that a martyr’s death will send them straight to heaven." - R. Dawkins
- "Where there is evidence, no one speaks of 'faith'. We do not speak of faith that two and two are four or that the earth is round. We only speak of faith when we wish to substitute emotion for evidence." - Bertrand Russell
- "And now these three remain: faith, hope and love. But the greatest of these is love." - 1 Corinthians 13:13
- "What do you want if you don't want money?" - Adam Faith
- "Reason is the greatest enemy that faith has." - Martin Luther