David Hume contributed the idea that all knowledge is based on experience to both the philosophy and the science of the enlightenment. He also argued against a priori reasoning, which was popular at the time.
Hume is best known for his empiricism, which states that all knowledge is based on experience. He thought that there were two kinds of knowledge: impressions and ideas. Impressions are sensory perceptions, such as sights, sounds or smells. Ideas are mental representations of these impressions, such as when we remember a certain sight or sound.
In addition to empiricism, Hume also contributed to another important philosophical concept: skepticism. Skeptics argue that it is impossible to know anything with certainty because we can never be sure if what we perceive is actually real or just an illusion created by our own minds. For example, when you look out your window at night and see a tree in your backyard, that may not actually be a tree at all — it could just be something you’re imagining in your head!
In science, Hume promoted the idea of a “regularity theory” for explaining events in nature (for example, why does lightning strike?). According to him, there must be some natural law at work causing lightning to strike when it does (such as when clouds get charged up with electricity).