We now have it on the sayso of the Pope that 'evolution is more than a theory'. His Holiness has come publicly to this conclusion long after most competent observers and many Christian thinkers had, effectively, accepted evolution as fact. So the Papal announcement, welcome as it is, cannot be said to have added much to our knowledge of the subject. The announcement is primarily a belated acknowledgment (more implicit than explicit) that empirical observations, and logical inference therefrom, are more dependable than perceived authority as means of comprehending the universe.

The announcement is something of a philosophical victory - a victory for empiricism (the notion that observed data are the foundation of knowledge and understanding) over rationalism (the notion that essential truths are either revealed or self-evident and that knowledge and understanding are to be arrived at by logical deduction from such truths).

Of course we are all, to some degree, both empiricists and rationalists but the Papal announcement must, beneath the immediate question of evolution, shift us towards empiricism and away from rationalism.

A moment's reflection reminds us that fanatical totalitarian tyranny has as its philosophical basis the idea that, come what may, the logic of its unquestioned assumptions is all that really matters - the lessons of experience are to be swept aside if those lessons do not fit the logical consequences of those assumptions.

The question remains 'Did evolution really happen - is it fact?'

It is easier to make factual statements than it is to decide dependably whether they are true or not. Some factual statements are demonstrably true. Try one; BT Directory Inquiries will tell you, as fact, my phone number; dial it and my bell will ring (unless the line is engaged or out of order etc); dial any other number and my bell will not ring. Some factual statements are demonstrably false - 'there is a horse in my bedroom' - a horse is so big and my bedroom is so small that were such a beast present the fact could not be missed by anyone.

But most factual statements that we find it convenient to categorise as true, or false (as the case may be) are not strictly known to be the one or the other. Most factual statements have a probability tag attached to them ranging from 'scarcely credible' to 'hardly deniable'.

How can we attach, dependably, a probability rating to a given factual statement. Guesswork .... 'I feel in my bones that it must (or cannot) be true' ... ask somebody to whom we attribute superior knowledge ..... these procedures have often deepened our ignorance rather than extended our knowledge. We need something better than such, essentially lazy, approaches as those.

Increasingly, people are coming to see the usefulness of 'conceptual models' as means of attaching probability ratings to factual statements. Most people are unaware of the phrase, conceptual model, but most thinking people tend to think that way without being conscious of the underlying procedures. Crime fiction usually revolves around conceptual modelling.

A conceptual model is the preferred scientific tool for understanding the data that the observer deems to be reliable and significant. (There are many assumptions behind those innocent-looking adjectives but that matter is best considered on another occasion).

The conceptual modeller performs an 'as if' exercise - an adult version of the pretend games of childhood. The modeller says, in effect, 'let us set aside the real world (whatever that reassuring phrase may be taken to mean) and instead construct an imaginary world in which the given data would be true'. IFF (short for if and only if) that imaginary world is BOTH a) not internally inconsistent (not incoherent) AND b) not at variance with new data coming to the modeller's notice (corresponds with the data) THEN, and only then, the model is a good one; it is 'as if' it were the real truth.

A good model is true enough to be taken as really true unless and until data come to hand that falsify it. If that happens then the model has to be amended or perhaps even scrapped.

It is important to understand that no conceptual model, however well it complies with the criteria of coherence and correspondence, can be taken to be finally, absolutely, true. There are two reasons for this; one is that unsuspected data may yet come to light; the other is that, in principle, different models may give equally good accounts of the necessarily limited data we have at any given moment.

Biblical literalists often claim that Genesis is absolutely true. Evolutionists do not, or at least should not, claim that evolution is known absolutely to be true. All they can, and should, claim is that evolution is the best conceptual model we have, so far, to cope with the huge volume of data we have in such areas as geology, fossil studies, comparative anatomy, embryology and many others.

To say that evolution is 'more than a theory' is to say that it is a satisfactory conceptual model - in active use and under continuous active review.

The Vatican now acknowledges that evolution is indeed a satisfactory model; most competent scientists have long thought that it is very satisfactory indeed.

or e-mail to stockton.sanday.orkney@zetnet.co.uk