Most people don't really question their senses, but most people are also familiar with the types of mistake reported above. Why, then, aren't we more sceptical of the information coming through our senses?

For convenience, we can categorise the type of error under four main headings:

1. Optical illusions
These are traditional examples of diagrams, pictures, models, etc., which seem to provide odd effects on the senses. Look at the following pictures and note what you think this could tell us about our senses:

(a) The Grid
Stare at the grid below for a few seconds. Can you notice anything strange happening?

(b) Sloping lines
Look at the picture below of sloping lines. Or are they?

(c) Ascending and descending
Are the monks walking up or down? (Drawing by M.C.Escher)

(d) Old or young woman?
Can you see more than one face here?

For further examples of illusions see:

http://www.optillusions.com

2. Delusions, waking dreams and visions

There is, as you might guess, a great deal of debate as to whether visions - in the religious sense - actually exist. However, setting this issue aside for the moment, it is possible to classify certain experiences as delusional. For example, someone who is running a fever, has suddenly woken from a deep sleep, is exhausted through hunger or fatigue, or even under extreme stress, may hallucinate.

We may also include here certain types of mental illness where voices are heard or things projected out onto the outside world so that they appear as real (whatever that is! We shall come back to this later). However, the main thing to notice here is that the mind is capable of producing illusions under certain circumstances.

3. Natural illusions  

The types of thing included here would be such things as: moisture rising from the ground appearing as a pool of water (a mirage); the light of a star that has by now died but can still be seen; a stick in water that looks bent; the way the moon looks bigger nearer to the horizon.

These, and other examples, are often cited as proof that the senses cannot in themselves be trusted. As with the optical illusions (above), there seems to be a natural tendency to misinterpret, or provide misleading information regarding certain experiences.

4. Relative or subjective sensations

Hot water to a cold hand can feel hotter than to a warm hand - and vice versa. Also, people who have had a limb amputated sometimes still have sensations where the limb was. Experiences of this type suggest that even something as fundamental as our bodily sensations can be mistaken.

This is an especially strong point for the sceptic because our sense of touch is very often seen as being the most reliable, the thing most capable of giving us proof. If we can touch or feel something we are more likely to accept it as real than if we have merely seen it.

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