This article is part of the ongoing series How Belief Works, which is best read in order.

We all know what it means to believe something, from the significant, such as the belief that humans are apes, to the mundane, such as the belief that David is vegetarian. Indeed, belief is one of the most basic concepts of human psychology. Yet it's also one of those everyday concepts that can be surprisingly difficult to define, like art or happiness.

At minimum, belief is a psychological state that involves a claim, such as Humans are apes or David is vegetarian.

Although a claim is asserted by a sentence, it hasn't necessarily been asserted by a person. For example, upon seeing it's raining, we can form the belief It's raining without anyone asserting this. So to say that belief involves a claim isn't to say that it involves something asserted by someone.

Also, the term belief can refer to either the psychological state or simply the believed claim. That is, it can refer to either believing something or something believed. In the first sense, my belief that David is vegetarian, and someone else's identical belief, are separate psychological states existing in different heads. But in the second sense they’re the same thing: the believed claim David is vegetarian.

So the term mundane belief refers to either belief of a mundane claim or a mundane claim that's believed – and likewise for the terms significant belief, true belief and false belief.

We actually often use the term belief by itself to specifically mean a significant claim that's believed, whether moral, political, religious, historical, philosophical or scientific. For example, when we refer to defending our beliefs, we're referring to defending particular significant claims we believe.

But the subject of How Belief Works is our belief of any kind of claim, however significant or mundane. Other examples of mundane beliefs are my current beliefs that I’m in my flat, that I'm sitting on a chair at my desk, that I'm typing these words, that it's Tuesday morning, that it’s sunny outside, and that I went for a hike along a river yesterday.

So what is the definition of belief in the sense of the psychological state? Given that belief involves a claim, the answer can seem as obvious as it is simple: considering a claim to be true. This definition can seem irrefutable, but it's fatally logically flawed – as I’ll explain in the next article, on how beliefs form.