Man is the measure of all things.
This is the most famous saying of Protagoras, though it is in fact, only the first portion of his statement. The full line runs ‘Man is the measure of all things; of things that are that they are, and of things that are not that they are not.’ Protagoras’ relativism is one of the most extreme ever argued. This means that truth is relative and for each individual truth is different. This can be true with things like temperature – you might find the evening chilly, but for me it is warm. However, we can all agree on the absolute temperature in degrees. Protagoras would disagree and would say that all of our knowledge is sense based and therefore unique to each individual. The problem with relativism is that it makes philosophical discussion impossible; what you think you say and what I hear might be completely different, if we are unable to agree on objective truth.