Niki asked:

How far can we know what is true? Because as I see it there is no half truth, we either know our world
or we are living a lie.

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There is no such thing as half-truth, but there is such a thing as knowing, or saying something which
is less than the whole truth. Courts of law are familiar with the concept of the whole truth: hence the
oath which requires the witness to state, "The truth, the whole truth and nothing but the truth." You
can say things which are true, but by your omissions, or by your choice of emphasis, lead your
listeners to conclude things which are not true. You have not said anything false, yet the effect is just
the same as if you had.

From the point of view of logic, a statement is either true or it isn't. To take the logician Tarski's
famous example, "Snow is white" is true if and only if snow is white. If I tell you that snow is white, or
if I tell you that "Snow is white" is true, I have conveyed exactly the same information. Generations of
students, meanwhile, have wondered how on earth one can make such a sweeping statement as,
"Snow is white" when everyone who has ever seen snow knows that it can be a multitude of shades!
There is no precisely defined point where something ceases to be white and becomes cream, or grey,
or indeed where snow ceases to be snow and becomes sleet, or slush.

Is it then not true to say that snow is white, but only truerto say that snow is white than to say it isn't?
What colour is snow, if not white? The thing to say here is that our language, with all its vagueness,
does preciselythe job that it is designed to do. We could not convey to one another what our senses
told us, if we were only permitted to use concepts with precise definitions. On the whole, our senses
are reliable witnesses, even though they fail to deliver scientific precision.

This case can be generalised. What I have said applies not only to knowledge gained by sense
perception, where we make judgements that such and such is 'large' or 'small', or 'white' or 'cream', or
a 'heap' or a 'pile', but also to knowledge which expresses a theoryabout the things we perceive, a
theory which perhaps works only as a first or second approximation. Human or 'folk' psychology,
which talks of beliefs, desires, intentions is held by some philosophers of mind to be only
approximately true, and by others to be false, though useful. But the same thing applies here as in the
case of vagueness. Take away folk psychology, and whatever 'scientific' account can be given of
processes going in the brain will never be an adequate substitute.

There is much more knowledge about the world to be had than the knowledge we possess, or even
seek. The world is a world of illusion only for those who mistakenly believe that all they know is all
there is to be known.

Geoffrey Klempner