Yet, despite their importance, one of the long-standing problems with studying beliefs is identifying exactly what it is you are trying to understand. “Everyone knows what belief is until you ask them to define it,” says Halligan. What is generally agreed is that belief is a bit like knowledge, but more personal. Knowing something is true is different from believing it to be true; knowledge is objective, but belief is subjective. It is this leap-of-faith aspect that gives belief its singular, and troublesome, character.
Philosophers have long argued about the relationship between knowing and believing. In the 17th century, René Descartes and Baruch Spinoza clashed over this issue while trying to explain how we arrive at our beliefs. Descartes thought understanding must come first; only once you have understood something can you weigh it up and decide whether to believe it or not. Spinoza didn’t agree. He claimed that to know something is to automatically believe it; only once you have believed something can you un-believe it. The difference may seem trivial but it has major implications for how belief works.
If you were designing a belief-acquisition system from scratch it would probably look like the Cartesian one. Spinoza’s view, on the other hand, seems implausible. If the default state of the human brain is to unthinkingly accept what we learn as true, then our common-sense understanding of beliefs as something we reason our way to goes out of the window. Yet, strangely, the evidence seems to support Spinoza. For example, young children are extremely credulous, suggesting that the ability to doubt and reject requires more mental resources than acceptance. Similarly, fatigued or distracted people are more susceptible to persuasion. And when neuroscientists joined the party, their findings added weight to Spinoza’s view.