When you think or say about something concrete/individual, then you still say it in abstract words but focused on one specific thing. Everything has it's essence/idea/form, it is being. But concrete things also have material part, but matter is only potence rather than actuality (true being). material/substrate is non-existence because what makes something exist - is essence rather than matter. So only essences/forms/ideas exist. Material (what promotes individuation) doesn't exist.