||Berkeley's immaterialism introduces a strict dichotomy between ideas and spirits (what we would call minds). While he spends much of The Principles and Three Dialogues discussing the nature of ideas, he does little to provide an explanation of the nature of spirits - and, in particular, how we gain knowledge of them. An especially troublesome area is the epistemology of other minds. Berkeley explains that we learn about our own minds by means of inward reflection, and that the existence of God's mind is evinced by the mechanisms of nature that we observe around us. However, other minds are under-discussed - particularly in light of the so-called 'likeness principle' in section 8 of The Principles which tells us that we can't perceive minds like we do other objects. In later editions of The Principles, Berkeley introduces the term 'notion' and suggests that while we can't gain perceptual knowledge of minds we can at least possess notional knowledge of their existence. However, the term 'notion' is also somewhat problematic.