In this chapter we analyse and evaluate an argument by a famous philosopher, A. J. Ayer, about the connection between (i) electrical impulses in our brains and (ii) thoughts and ideas in our minds. Most of us have a reasonably clear idea about what we mean when we talk about ‘thoughts and ideas in our mind’ and we know that scientists study how the brain works, how it is constructed, how ‘messages’ are transmitted within it and so on, but Ayer's piece is about the connection, if any, between the two – about how one's brain and mind interact with each other.

If you conjure up an image of an elephant in your mind's eye, or entertain the thought that ‘the moon has a weak gravitational field’ or decide to raise your arm, you might expect some ‘corresponding’ activity in your brain – some activity (physical, chemical, electrical or whatever) of a kind that scientists could see, describe and study. But what is the connection between the two? Does one cause the other? Does the ‘mental activity’ of deciding to raise your arm somehow send signals to your muscles which cause your arm to raise and, conversely, do electrical impulses in your brain somehow cause you to have the image of an elephant in your mind's eye or to entertain the thought about the moon's gravitational field? Alternatively, do these two distinct kinds of activities – the mental and the physical – just occur together without causally interacting?