Because ‘thickness’ always leaves room for dispute, thisis one of those cases in philosophy in which one is at the mercy of thearguments philosophers happen to think up. The conceivability argumentcreates a prima facie case for thinking that mind has no more thancausal ontological dependence on the body. Let us assume that onerejects analytical (behaviourist or functionalist) accounts of mentalpredicates. Then the above arguments show that any necessary dependenceof mind on body does not follow the model that applies in otherscientific cases. This does not show that there may not be otherreasons for believing in such dependence, for so many of the conceptsin the area are still contested. For example, it might be argued thatidentity through time requires the kind of spatial existence that onlybody can give: or that the causal continuity required by a stream ofconsciousness cannot be a property of mere phenomena. All these mightbe put forward as ways of filling out those aspects of ourunderstanding of the self that are only obliquely, not transparently,presented in self-awareness. The dualist must respond to any claim asit arises: the conceivability argument does not pre-empt them.
It may turn out that the argument-based approach to necessitydeployed by modalism is limited in that many extraordinary modalclaims, claims for example about the necessity of origins, are suchthat there is no secure ground for them, since they rest oncontroversial metaphysical principles. By contrast the approach maydeliver many ordinary modal claims, claims that are natural extensionsof well-grounded scientific and mathematical theories.
Human Knowledge: Foundations and Limits
E.J. Lowe (2008a, 2012) and Bob Hale (2013) have independentlydeveloped accounts of the epistemology of modality based onmetaphysical essentialism. The two core theses of metaphysicalessentialism are: (i) entities have essential properties oressences that are not merely dependent on language, and (ii) not allnecessary truths capture an essential truth or the essence of anentity. Although their views differ at crucial points in the epistemiclandscape, the program they share maintains the following:
Definition of Modality, Philosophy and Metaphysics of ..
It is prima facie plausible to think that all modal knowledgeis in principle a priori, since at least perception ofactuality cannot provide one with knowledge of mere possibility andnecessity. For example, if conceivability is taken to be an apriori exercise, and it is linked to possibility, then it isplausible to think that a priori conceiving that Pprovides one with a priori justification for believing thatP is possible. Likewise, finding P inconceivableprovides one with a priori evidence that P isimpossible. While this might seem to be the only way thatsuch knowledge can be discovered, this simple thought is challenged byKripke’s arguments for the existence of a posteriorinecessities. The problem is discussed in detail in Yablo’s(1993): Is Conceivability a Guide to Possibility? One of themain problems facing contemporary a priori accounts of theepistemology of modality concerns the existence of a posteriorinecessities. Recall that an a posteriori necessity is astatement, such as the identity statement “Water =H2O”, that is metaphysically necessary, yet knowableonly a posteriori. As a consequence, a prioriaccounts face the following potential situation:
Essays in the Metaphysics of Modality
One reason to accept (b) is that metaphysical possibility andnecessity, as defined as truth in some possible world and truth in allpossible worlds may itself reduce either to logical possibility andnecessity or physical possibility and necessity. For our appearancesof possibility and necessity to be about metaphysical possibility andnecessity it must be the case that the best explanation is that thereis a unique kind of modality picked out by “metaphysicalmodality” and that this modality is the best explanation forwhat our appearances of possibility and necessity are really about. Ifmetaphysical modality collapses either into logical modality orphysical modality, then there is no reason to believe that ourappearances of possibility and necessity are really about metaphysicalmodality.