– The starting point of idealism, at least in the modern period, can be traced to Descartes and his distinction between material and immaterial substances. Material substances are foreign to immaterial substances – one cannot know or cause or move the other.
– From that, it follows that sensations are either innate (Descartes) or given by God (Malebrance), since sensations can’t be produced by material substances.
– Malebrance developed Descartes logic with regard to causality – since we have no clear and distinct idea of causality to apply to matter, he fell back on God to supply causality.
– The obvious criticism here is Hume’s: if we have no idea of causality to apply to matter, how on earth can we apply it to God? Hence Hume’s position of causality being our own ideas projected onto objects, learned from custom.
– Berkeley’s critique can also be seeing hovering in the background: if sensations are explained entirely by the mind/God, why suppose ‘matter’?
– Descartes definition of matter and material substances had the effect of producing a world full of mutually exclusive substances which cannot act on or be acted upon each other. Matter has only mobility, not motion or the power to cause motion. God is the cause of all motion.
– The great sceptical problems are seen to be necessary conclusions if Descartes principles are accepted – matter as extension, the exclusivity of substances, the location of sensations within us as innate ideas, etc.