It is the customary fate of new truths to begin as heresies and end as superstitions. (T H Huxley)

We talk of truth, but what do we mean by truth? Felipe Fernandes-Armesto has suggested that there are four kinds of truth:

The truth you feel is the truth you know in your guts. It is the truth of instinct and intuition. It includes truths acquired through spiritual experiences and personal revelation.

The truth you are told is the truth of tradition, of the oracle, of the dream and of the prophet. It is the truth communicated from other worlds via some form of intermediary: the priest, the prophet, the medium and the shaman. It is the truth of sacred texts such as the Bible and the Koran.

The truth of reason is the truth of rational thought and logic. It is the truth arrived at by testing the accuracy of a claim by applying a set of precise rules. It is the truth of pure thought, independent of input from outside of the mind. It is exemplified by mathematics.

The truth you perceive through your senses is the truth arrived at through experience, observation, recorded facts and experiment. It includes the empirical truths found by science.

Truths are sometimes compound in nature. For example an historic truth might be drawn from an ancient chronicle (a truth that you are told), but be supported by an archaeological finding (a truth you perceive). Truths can also change their category. An scientific intuition (a truth you feel) might by experimentation become a truth you perceive.

The definition of truth is something that accurately describes reality. What complicates this is that there are two candidate realities. The first is the reality that exists in our minds, the reality of our thoughts and emotions. The second is the reality that exists outside our minds, the reality of the external world.

The first form of reality, our internal reality, we know exists. Its truths are personal and subjective. If we were happy to accept all truths as just being personal and subjective, life would be simpler. What is true for me would not have to be true for you, and vice versa. However, if we accept that there is a reality external to our minds, then its truths are independent of us. They are objective.

As soon as we try to assert that something is objectively true we encounter problems. For the first two types of truth, the truth you feel and the truth you are told, the difficulties are obvious. The truth you feel is of its very nature subjective, while the truth you are told relies on your accepting the authority of the messenger. Believers of different traditions may assert the authority of Jesus or Mohamed, but for those who do not share their commitment, what appears certain is the faith of the believer, and not the message.

To say that we cannot tests such truths objectively if of course not to say that the are necessarily untrue, or even that they have no practical use. For example instinctive truths have often to be relied on when evidence is lacking or time is short. Furthermore, as we lack the time and skills to establish all truths by logic and observation, in real life we all rely upon authorities, for example Newton, Darwin, Einstien etc. to do this for us.

Because of its apparent lack of objectivity, holders of religious beliefs have often tried to substantiate their beliefs by using arguments based on reason and the evidence of the world around them. However, sadly for all seekers after truth, neither the truth you reason nor the truth you perceive offer any guarantees of accurately describing the real world.

The truth of reason is a mental construction that starts with one or more initial assumptions, called axioms, from which conclusions are deduced. For example: " All men are mortal. Socrates is a man. Therefore Socrates is mortal." The axioms are taken to be true, but in our search for objective truth we cannot accept anything as being self-evidently true. We need to prove the axioms.

Two other weaknesses in all systems of belief constructed from logic have been demonstrated by the mathematician Kurt Godel in what is known as Godel’s incompleteness theorem. In 1931 Godel demonstrated that within any given branch of mathematics, there would always be some propositions that couldn't be proven either true or false using the rules and axioms of that mathematical branch itself. You might be able to prove every conceivable statement about numbers within a system by going outside the system in order to come up with new rules and axioms, but by doing so you'll only create a larger system with its own unprovable statements. The implication is that all logical system of any complexity are, by definition, incomplete; each of them contains, at any given time, more true statements than it can possibly prove according to its own defining set of rules. Godel also demonstrated that if a system is consistent, the consistency of the axioms cannot be proved within the system. In other words we cannot prove that the axioms aren't contradictory.

The fourth kind of truth, the truth that we perceive, relies upon us observing the world about us. This gave us the knowledge to navigate by the stars, and to build the pyramids. It is the bedrock of the scientific process, in which theories are produced that are consistent with the observations, which provides explanations for them, and from which new ones can be predicted.

The ultimate goal of science is to establish universal truths (laws of nature) which unlock the secrets of the universe. Such laws use empirical observations not just to prove that they are correct, but in their axioms and as constants in their equations. However, it may be argued that a theory which requires empirical observations, it is in an important way incomplete. If through its own logic it does not prove its correctness by showing that reality could not be otherwise, then it cannot tell us why it is true. It cannot tell us why reality is this way, rather than some other way.

A more general problem with 'the truths that we perceive' is that they assume that there is a world about us to be observed, that the information we receive about it through our senses is reliable, and that what happened in the past will continue to happen in the future. All these assumptions are problematical. The first two, the nature of reality and reliability of perception, are dealt with in the essay on reality. Suffice it here to say that it is impossible to answer with absolute confidence the question, "What exists?"

The third assumption, that the future will resemble the past, is of course un-provable. Within human memory the sun has always risen towards the east, but one day it may swallow up the earth and rise no more. This demonstrates a more general problem with ‘truths’ arrived at by induction, by inferring from the particular to the general. There is no absolute justification for them. Some will be correct, and others not.

Yet another difficulty in the search for objective truth rests with the nature of language and the mind. Why should we believe that our minds are capable of embracing the truth? We do not assume that our pets are capable of understanding calculus, why should we be capable of understanding the universe? Many philosophers and scientists believe we think in terms of language, either our natural language or some form of in-built generic language, ‘mentalese’. In either case, our ability to think may well be limited by the language we think in. Furthermore, to the extent that we communicate an objective truth in terms of language, we are dependent on the words we use, but the meaning of such words can only be defined in terms of other words. Words, unlike mathematics, can never be precise.

From all this flows the view held by certain modern philosophers (or more correctly ‘post-modern’ philosophers, the schools of thought that emerged in the 1960s) that all truth is subjective, relative and of equal merit. The only certainty is the existence of self, of subjective self-awareness, and the attempt to establish any universal truths must be abandoned, in effect that truth is socially constructed. The problem with this position is of course that it does not correspond with our experience of the world, which whether real or imaginary, we have to cope with. In the world as we experience it certain truths correspond much more accurately with our experience, and have a much greater utility than others. Even our post modern philosophers will prefer an electrician who subscribes to the view that equipment should be properly earthed. More generally we mights say that for most of our daily life we adopt a pragmatic view of truth. A truth is confirmed by its effectiveness when applying it to actual practice, or because it bestows some other benefit- social, psychological, spiritual etc.

So where does this leave us? Well perhaps at least with an understanding that:

• When we speak of truth we need to understand what kind of truth we are talking about.
• That all truths, except perhaps the truth that the mind experiences thoughts and emotions, rest on assumptions that are ultimately unprovable.
• That for all of us deciding on what we believe to be true is largely a matter of deciding what authorities we accept.
• That those of a scientific or philosophical disposition are likely to choose authorities that arrive at their truths by logic and empirical observation.