In 1985 Daniel Garber published his highly intluential paper “Leibniz and the Foundations of Physics: The Middle Years”. In two recent articles, Garber returns to these issues with a new position - that we should perhaps conclude that Leibniz did not have a view concerning the ultimate ontology of substance during his middle years. I discuss the viability of this position and consider some more general methodological issues that arise from this discussion.

This paper addresses D. C. Williams’s question, “How can Leibniz know that he is a member of the actual world and not merely a possible monad on the shelf of essence?” A variety of answers are considered. Ultimately, it is argued that no particular perception of a state of affairs in the world can warrant knowledge of one’s actuality, nor can the awareness of any property within oneself; rather, it is the nature of experience itself, with the flow of perceptions, that guarantees our actuality. A consequence of this view is that no non-actual individuals can truly be said to experience their worlds, nor can they ask the question if they are actual or not.

In notes from 1675-76 Leibniz is using the notion of an infinite number as an illustration of an impossible notion. In the same notes, he is also using this notion in contrast to the possibility of the ‘Ens perfectissumum’ (A.6.3 572; Pk 91; A.6.3 325). I suggest that Leibniz’s concern about the possibility of the notion of ‘the greatest or the most perfect being’ is partly motivated by his observation that similar notions, such as ‘the greatest number’, are impossible. This leads to the question how Leibniz convinced himself that the notion of the greatest number is self-contradictory and that of the greatest being is not. I consider two suggestions, one that stress the difference between beings and numbers and one that stress the difference between two notions of infinity, and conclude that neither of them provides a satisfactory solution to this question.