Why Leibniz Should Have Agreed with Berkeley about Abstract Ideas

Leibniz claims that Berkeley “wrongly or at least pointlessly rejects abstract ideas”. What he fails to realize, however, is that some of his own core views commit him to essentially the same stance. His belief that this is the best (and thus most harmonious) possible world, which itself stems from his Principle of Sufficient Reason, leads him to infer that mind and body must perfectly represent or ‘express’ one another. In the case of abstract thoughts he admits that this can happen only in virtue of thinking of some image that, being essentially a mental copy of a brain state, expresses (and is expressed by) that state. But here he faces a problem. In order for a thought to be genuinely abstract, its representational content must differ from that of any mental image, since the latter can represent only particular things. In that case, however, an exact correspondence between the accompanying mental image and the brain state would not suffice to establish a perfect harmony between mind and body. Even on Leibniz’s own principles, then, it appears that Berkeley was right to dismiss abstract ideas.