Here are the main points of the arguments that lead us to this conclusion. When we infer the existence of one object from the existence of others, some object must always be present either to the memory or senses to serve as the foundation of our reasoning (the alternative being a regress ad infinitum). Reason can never satisfy us that the existence of any one object ever implies the existence of another; so when we pass from an impression of one to an idea of or belief in another, we are driven not by reason but by custom, or an associative force. But belief is something more than a simple idea. It is a particular manner of forming an idea; and as an idea can be varied—·without being turned into another idea·—only by a variation of its degree of force and liveliness, it follows from all this that belief is a lively idea produced by a relation to a present impression, which is the definition I gave.