It was just another Friday morning lecture with a ragged row of students slumped in chairs pointing their large boots in my direction, when one of them suddenly blurted out: "I never thought of that before," right in the middle of my sentence.

I can see the look of mixed bafflement and joy on his face and I know it is one of those moments that makes it all worth while. In my confusion, I laugh and say: "That is supposed to happen in lectures, you know." But I am secretly dead chuffed.

This was the third-year course on the psychology of consciousness and we had just been discussing Francis Crick's "Astonishing Hypothesis". I asked them why they had heard of Crick - hoping to goodness they had, and fearing, in the long silence that followed, that either I was getting too old or they really were plain ignorant. But no, someone mutters "genes?", another "DNA", "The double helix?" Right, the Nobel laureate, co-discoverer of the structure of DNA, has now proclaimed the "Astonishing Hypothesis" that you are nothing but a pack of neurons.

Crick's book argues that consciousness is not something special or different from brain processes. To understand consciousness, all we need to do is study the way the brain builds visual representations of the world, processes emotional information, or constructs plans and hopes. All these things, the very stuff of our inner lives, are nothing more than the firing of millions of little nerve cells, all connected up in especially clever ways.

"Why did he say, 'You're nothing but a pack of neurons"?" I push on. I wonder whether it is worth all this fishing for allusions. Should I be spending more of our valuable hour on the nitty-gritty of those neural connections?

But they're getting there ... more muttering ... "Alice!" says one skinny girl, who looks about 15 but is extraordinarily bright.

Yes. In the courtroom of the King and Queen of Hearts, judge and jury are about to pronounce sentence on the poor old knave, accused of stealing the tarts. "Off with her head," shouts the Queen at Alice, and Alice, who has finally had enough, retorts: "Who cares for you? You're nothing but a pack of cards!" And with that, awakens from her curious dream.

The point is that the courtiers are going to live or die. They care, they fear, they plead. It matters to them. Of course they are only cards, and we all laugh. But what if we - and all our cares, fears and pleadings - are only a pack of neurons? What then?

We can think about this intellectually and it fails to touch us. We can argue philosophically or study the details of neuronal processes, without taking it into our lives. Or we can realise head on: that means me. What I saw in that first student - the mature student (whoops, sorry, I mean non-standard entry student) - was a look on his face that meant he'd seen it. He had actually grasped the possibility that everything he held dear might be the mindless construction of a clever brain. No wonder he blurted it out.

Of course, he doesn't yet know that next week we'll be tearing Crick to shreds. Of course, consciousness isn't just the firing of neurons. What about subjectivity - isn't there a "hard problem" of consciousness that Crick simply ignores? "Yes, yes," some of them will say. "We don't want to be reductionists!" So we'll struggle with the hard problem until we realise that we don't know what that is - and so we'll go on. And maybe more of them will take the problems to heart, and I will feel it's all worth while.

But what about the responsibility of it all? I have seen students quite frightened by the implications of what we are discussing. Suddenly the cosy world of their childhood religion can stand no more and is torn apart. Suddenly they look inside themselves and see there is nobody in there. My own view is that this is what psychology should be all about. There is no point being interested in the mind if you daren't face up to what science seems to be telling us about it. However, I sometimes get the impression that we lecturers are expected to deliver a package, a nice, safe package, with a price on the label; a parcel of knowledge that you, the student, can acquire and take away with you when you leave.

So to salve my conscience, I warn them right at the start, in their course handout: "Warning - this course may change your life." But by the time they understand the warning, it's too late. They've started to think.

The author is senior lecturer in psychology at the University of the West of England. Her most recent book, an autobiography, 'In Search of the Light: the Adventures of a Parapsychologist', is published by Prometheus, pounds 14.50.