  As to (2), Reid thinks that nobody who has absorbed Hume's lessons regarding causation would think that we can avoid skepticism about the external world while insisting that we infer its nature from the features of directly perceived ideas. On the representational model, external objects are the causes of our ideas. But if Hume is right about causation then we can only infer the nature of a particular unobserved cause of a particular observed effect when we have had repeated experience of conjunction of similar causes with similar effects. But according to the Way of Ideas we have never in fact had any experience of the relevant causes, namely physical objects and their qualities; we have only experienced their effects on our minds, ideas. Therefore, we can infer nothing about external objects by examination of the ideas which they cause in us. Option (2) appears to give skepticism a foothold.