The blog Conscious Entities has an interesting post on Idealism and also one on the moral, intellectual, and phenomenal gaps between animals and humans.
Idealism is the view that everything we encounter in the world is ultimately reducible to ideas in minds. There is no material substratum to reality.
It was given its most notable expression by George Berkeley (1685-1753) in Principles of Human Knowledge where he argues that all we can experience are sensory phenomena like color, fragrance, sound, etc. But these sensory phenomena exist only in minds, thus all we can have experience of are ideas in our minds. It follows that if knowledge is a product of experience then all we can know are these ideas. A material "stuff" that generates the phenomena of experience is not only absurd but unknowable.
Since Berkeley was an empiricist and believed that all of our knowledge comes through sense experience, he concluded that we can know nothing of anything other than these ideas in our minds. We have no experience of, and therefore no knowledge of, matter. The notion that there is a material world behind the phenomena of our senses is superfluous and should therefore fall victim to Occam's Razor.
All that exists, according to Berkeley, are ideas in minds, and for Berkeley the entire world exists as idea in the mind of God. The world is a virtual reality generated by God, which may be one way to read Hebrews 1:3 and Colossians 1:13.
RLC