Tonight's speaker for Glasgow Skeptics, Matt Dillahunty, defines faith as: 'The excuse people give when they don't have a good reason'. Mark Twain similarly referred to faith as: 'Believing what you know ain't so'. Religious adherents, however, will say that faith is a reasoned position and that we all exercise faith when taking decisions based on incomplete evidence. They will often point to a chair and say that we have faith that the chair will support us, they may say we have faith in a policeman or a lollipop man (this was this writer's definition given at catholic school) or they may ask us if we have faith our partner is not deceiving us behind our back. Having faith without a good reason is dismissed as 'blind faith'.
But, this equivocates two meanings. Faith in one context is synonymous with trust. And trust is built up after a period of experience to form a reasonable expectation of future events. That trust can be dented or challenged by new information or evidence: a damaged chair leg; smelling alcohol on the breath of a school crossing person, or suspicious, unexplained behaviour by one's partner. Religious believers use faith not as reasonable trust or expectation but as the ultimate reason to believe. Even the Bible accepts this definition: 'Faith is the substance of things hoped for, the evidence of things not seen' (Hebrews 11:1).
How often have we heard or been involved in a debate with a theist who's ultimate retreat is "You just have to have faith". For skeptics, faith is the opposite of a reasoned evidential conclusion. Faith is accepting a proposition as true simply because we want it to be true. Think about where it is used; it always describes a state of being that we think is positive. 'I have faith my team will win the game', 'I have faith my God will answer my prayer', 'I have faith my life will turn around' etc. We never have faith something bad will happen. This is not to say that a positive outlook, or hope for a good result in an an unknown outcome is somehow irrelevant, they are often how human beings thrive and make us want to explore and face the world, but accepting something as true simply because we want it to be so and ignoring contrary evidence is not going to help us do so.