 The Justification for Implicit Knowledge Question: Sonia Roca-Royes (2010) draws attention to the following problem in Peacocke’s epistemology of modality. On Peacocke’s account, it appears that our knowledge or understanding of modality is parasitic on our knowledge of the Principles of Possibility, which are the principles anyone who possesses the concept of possibility tacitly knows. But how do we know such principles in the first place, or, equivalently, how does one manage to possess the right possibility-concept? We determine that something is possible or necessary for an entity in part through our tacit knowledge of them, which includes knowledge of what is constitutive of the entity, namely what it is to be the kind of thing in question. As a consequence of the relation between the role of constitutive principles and our evaluation of specific modal claims for the purposes of generating modal knowledge, a comprehensive account of modal knowledge is incomplete without a picture of how we come to know the relevant constitutive principles involved in our evaluations of modal knowledge. Peacocke has taken up this challenge in recent work (2020). More recently, Roca-Royes (2019) has generalized similar concerns to any account of modal knowledge that rests on concept possession and understanding.