Idealism

This theory was proposed by Bishop Berkeley as a theory of perception that tried to improve on representative/indirect realism. 

Berkeley denied the material world, and claimed that all objects are merely ideas within the mind. Ideas do not represent objects, they are objects.

He argues that we cannot have objects of perception without a perceiver. 

Esse est percepi = To be is to be perceived

The very idea of the material world goes beyond the boundaries of what we can experience, it's unknowable and therefore not a real concept. 

Berkeley pointed out that the distinction between primary and secondary qualities is wrong because the primary qualities also depend on the perceiver (e.g. the size of a building changes depending how far away you are from it).

If objects exist only when being perceived, then what happens when we stop perceiving them? Do they go in and out of existence? This seems absurd because evidence shows that objects exist and change when unperceived (e.g. a fire keeps burning even when it is not being perceived, or Bertrand Russel uses the example of a cat getting hungry).

Berkeley's solution was to say that objects are never unperceived: they exist always in the mind of God as the 'ultimate/permanent perceiver'.

• The reliance on God: the introduction of God to the argument is a problem for the non-believer. Also, idealism is supposed to be an empirical theory, but there is no experience of God. 
  - However, Berkeley used the argument to prove God, he didn't use God to prove the argument. 
• If all objects are in the mind of God, then how can God be immutable when things in the world grow, change and die?
• The logic of idealism is circular.
• Couldn't all objects be a figment of my imagination? What is the difference between a 'real' object and an imagination?
  - Berkeley says there are three main differences: real ideas are not voluntary, real ideas are coherent and they follow the laws of nature.
• Idealism leads to the trap of solipsism since we can't prove the minds of others. 

Ockham's Razor theory says that when we are faced with opposing explanations for something, the simplest is usually the best. 

But how do we apply this to perception?

The realist would claim that it is much simpler to refer to a material world than to involve God. The naive realist would also claim that there is no need to include sense data - the simplest explanation is that objects exist and we experience them directly.

But the idealist would claim that it is simpler not to refer to a material world that we have no experience of.