THE CENTRAL FACTS FROM THE COURSES YOU ALWAYS MEANT TO TAKE, IN 25 LECTURES

Metaphysics was traditionally about life, the universe and everything. The Greek philosophers began the search for a convincing answer to the question: what, if anything, underlies the world as we know it? And philosophers since, whether idealists or realists, theists or materialists, have taken up this question.

But one of the most famous of 20th-century philosophers taught that metaphysics is impossible, a futile exercise in pondering the imponderable and ending up talking nonsense. In his book Language, Truth and Logic, published in 1936, AJ Ayer argued that metaphysical speculation is literally meaningless. The big questions about the existence of God, of purpose in the world, of an overarching explanation of what it's all about are simply beyond our ken; they are not worth talking about.

Ayer had just returned to Britain from his exciting conversations with the Vienna Circle, a group of philosophers known as Logical Positivists. They taught young Freddie to recite the Verification Principle, which, let it be admitted, is itself a bit of a conundrum. It says: "The meaning of a statement is its method of verification." Which being translated reads: If you want to know whether something is the case, step outside and take a look. Except, that is, for mathematics, in which case you don't have to step outside; you just know that mathematical propositions are true by the meanings of the symbols themselves. In the jargon, they are said to be "analytic". Everything else you verify by observation.

Of course, when it comes to observation, you have to know where to look. We know where to look for tables and chairs - or at least for what Ayer called "sense-data" of them. But where do you look for such as God or purpose or the meaning of life? According to Ayer and the Verification Principle, we don't know where to look for these things. We don't know what sort of evidence would count for their verification.

A whiff of Logical Positivism has hung about ever since Ayer's day. Most people regard metaphysics as one of the childish things which the brave modern world has put away. But a few philosophers did reply to Ayer, asking the inconvenient question: "Hang on a minute - this Verification Principle of yours - how do we go about verifying it?" You can't verify it by the only means of verification that the principle itself allows. And so there are no Logical Positivists left nowadays.

The lesson is that we can't help doing metaphysics, one way or another: even the Verification Principle itself is a metaphysical doctrine because it sets out the limits to what can be known.

So where should we turn for an exposition of metaphysical philosophy? Before you try the rest, try the best.

Plato (428-347BC) was perhaps the first systematic metaphysician, and he suggested that reality is hierarchical. It begins with the lowest of phenomena, such as dreams and fantasies, and progresses upwards on the ladder of being through physical objects towards mathematical objects and finally to ideas or Forms, which, for Plato, are the perfect examples of things (eg, tables and chairs) and qualities (eg, colours). The highest reality of all is the Form of the Good.

When the Middle Ages rediscovered Aristotle, a great metaphysical row broke out. This was called the Realist/Nominalist controversy. Is there such a thing as redness apart from red objects, or are general terms only names? William of Ockham (1285-1349) was something of a referee in this squabble - though actually he was rather partisan in favour of the Nominalists. He gave philosophers a maxim which was known as Ockham's razor, Entia non sunt multiplicanda praeter necessitatem - roughly, Don't have more things in your metaphysics than you need.

The row rumbled on for centuries, until Kant (1724-1804) came along and changed the rules. The gist of Kant's Critique of Pure Reason is: Forget all this pretension to know the meaning of life, the universe and everything - for you can't know these things. Humans understand the universe as a snail understands his garden. In other words, we can only understand what we are equipped to understand. For Kant, these things were mere appearances, phenomena; we cannot see through appearances to the noumena, how things are in themselves.

It was just this view - that we could somehow see reality clear - that made the Logical Positivists see red, so to speak.

The post-war Existentialists also regarded all metaphysics as a kind of personal affront. Jean-Paul Sartre, for example, said metaphysics is impossible because it deals with essences, whereas the reality is that "my existence precedes my essence".

Maybe FH Bradley (1846-1924) displayed the right sort of philosophical phlegm about these matters when he said: "Metaphysics is the finding of bad reasons for what we believe upon instinct."

Tomorrow: Logic