Sensation and reality

Locke argued that objects have ‘real’ or primary properties such as mass or size. These are overlaid in or minds by our perceptions of them – their secondary qualities such as weight or appearance. Bishop Berkeley argues that perceptions are internal rather than external; when we see a banana its yellowness is part of our perception of it and not in the banana itself; Locke argued that what caused the yellowness was a real part of the object.

Berkeley attacks the idea that secondary qualities can exist in the object in a number of ways:

Same quality different sensation. If you hold and ice cube and then put your hand in some tepid water the water feels warm; if you put your hand in very hot water and then into the tepid water the tepid water will feel cold. Since the tepid water cannot be hot and cold at the same time it follows that the perception of temperature must be in the perceiver.

Taste. If I taste an apple it only has a taste at that moment; we cannot say that an untasted apple has a taste. It follows that some qualities only exist when being perceived.

Colour. If two people see the same object from different viewpoints, one might think it was a different colour to the other; since both colours cannot exist in the object at the same time, the colour must exist in the perceiver and not in the object.

Speed. If I stand still as a train passes, the people on the train will appear to be moving; but the people on the train see themselves as still and me as moving. If speed is in the object, how can the people on the train be both moving and see themselves at rest? The quality of speed must be in the perceiver. (also think of the opposite sensation, sometimes you can be stationary next to a moving train and think you are moving).

Size. A coin held near my eye will be bigger than sun. The size, and shape, of objects changes constantly according to the position of the object relative to the perceiver.

Berkeley’s arguments are convincing from a perceptual point of view; no matter how ‘real’ a quality might be, we only ever perceive it through our senses. We only ever deal with what Locke would consider secondary qualities.

Berkeley's goes on to argue that it is impossible for something to exist without being perceived: that is:

esse est percipi, (Latin "To be is to be perceived").

This means that we cannot really say that something exists unless we can either perceive it or imagine perceiving it. This represents an attack on thinkers like Plato or Isaac Newton who imagine that objects exist independently of human perception.

As far as Berkeley is concerned there is no access at all to the objects of perception.