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.

The aim of this paper is to trace in Leibniz’s drafts the sketched outline of a conceptual framework he organized around the key concept of ‘requisite’. We are faced with the project of a semi-formal theory of conditions, whose logical skeleton can have a lot of different interpretations. In particular, it is well suited to capture some crucial relations of ontological dependence. Firstly the area of ‘mediate requisites’ is explored - where causal and temporal relations are dealt with on the basis of a general theory of ‘consequence’.Then the study of ‘immediate requisites’ is taken into account - a true sample of mereological inquiry, where Leibniz strives for a unitary treatment of part-whole relation, conceptual inclusion and inherence. Far from simply conflating these relations one with another and with causality, therefore, Leibniz tried to spell them out, while at the same time understanding them within a single conceptual framework.

Leibniz’s physicalism about colors and other sensible qualities commits him to two theses about our knowledge of those qualities: first, that we can acquire ideas of sensible qualities apart from any direct acquaintance with the qualities themselves; second, that we can acquire distinct (i.e., non-confused) ideas of such qualities through the development of physical-theoretical accounts. According to some commentators, however, Leibniz frequently denies both claims. His views on the subject are muddled and incoherent, they say, both because he is ambivalent about the nature of sensible qualities, and because he gets confused about confusion, losing sight of his own distinction between the confusion proper to perceptions and that proper to ideas. In opposition to this, I argue that the critics have misunderstood Leibniz’s views, which are both consistent over time and coherent. The key to understanding his position is toappreciate what he characterizes as a kind of redundancy in our ideas of sensible qualities, a crucial feature of his view overlooked by the critics.

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.

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.