The association of ideas is the faculty that connects our ideas to one another. Nothing in the world is isolated, and so it is with our ideas. But this isn't to say that the association of ideas ever acts by chance. There's always a reason why two ideas seem to call out for one another. Here people often mention a story told by Hobbes. In the midst of a conversation about Charles I, someone asks about the value of the Roman denier under Tiberius.

The association of ideas guarantees the continuity of our intellectual life. Because ideas are associated with one another, the life of the mind is continuous – one idea calling for another, and so on indefinitely. Even when the activity of the mind is suspended, it continues to connect ideas unconsciously. In dreams, for example, the self no longer regulates the succession of ideas, but this succession continues unabated. Neither does all sensory communication with the outside world cease during sleep, for while the nervous system is at rest, it still transmits communications from the outside. These bring ideas – more or less conscious – into the soul, where they are mixed into the stream of other ideas.

In fact, the association of ideas continues even when we have blackouts, so there's no gap in the life of the mind. Although we don't have any experimental proof of this, it's incomprehensible that mental activity might cease only to start again a moment later.