Manifest kinds are natural kinds designed by terms like – water, tiger, gold, red, heat, electricity, -- individual instances of which are objects of our potential acquaintance about which we may have de re knowledge. Natural kinds of a more highly theoretical sort – like photons and neutrons – are not included in this category. Manifest natural kinds, or manifest kinds for short, figure in interesting statements of theoretical identification, many of which are both necessary and knowable only aposteriori. The aim of this paper is to explain how why this is so.
The necessity of many of these statements follows from their truth, plus the semantics of the natural kind terms they contain. Their aposterioricity requires a more complex explanation, which begins with the observation that our knowledge of manifest kinds parallels our knowledge of individuals. Just as our de re knowledge of individuals standardly depends on either our own acquaintance with those individuals, or the acquaintance of others who pass their knowledge on to us, so our de re knowledge of manifest kinds standardly depends on either own acquaintance with members of those kinds, or on the acquaintance of others who pass their knowledge on to us. Because of this requirement on acquaintance, most of our knowledge of individuals and of natural kinds can only be aposteriori. It is not possible to circumvent this result by using descriptions to semantically fix the reference of names or natural kind terms. In both cases, strong epistemic constraints on the use of a description D to semantically fix the referent of an expression E render our knowledge of the propositions expressed by E is D or E’s are D’s aposteriori, rather than apriori.
Epistemic constraints on reference fixing also restrict the possible explanations of our knowledge of necessary aposteriori truths like Water is H2O. These restrictions have especially important consequences for weak versions of two-dimensionalism according to which (i) some propositions (secondary intensions of sentences like Water is H2O) are both necessary and knowable only aposteriori, but (ii) our knowledge of these propositions (secondary intensions) is the result of our knowing that certain meanings (e.g. the meaning of Water is H2O) express truths in our context. A crucial feature of theories of this sort is their analysis of manifest kind terms like water as having their reference semantically fixed by description. Among other problems, these theories face an intractable dilemma: if they analyze the secondary intension of Water is H20 as being that expressed by The actual kind D is H2O, then they cannot give the truth conditions of sentences in which attitude ascriptions containing Water is H2O are embedded under modal operators. On the other hand, if they analyze the secondary intension of Water is H2O as that given by dthat [D] is H20, then the constraint on acquaintance makes knowledge that the meaning of the sentence expresses a truth insufficient for knowledge of the kind water that it is H2O. Either way the theory fails. These problems do not arise for strong versions of two dimensionalism – which maintain that what is reported by x knows that water is H2O is not that the agent knows the secondary intension of Water is H2O, but only that the agent knows that the meaning of the complement sentence expresses a truth (or something equivalent to that). However, this is not help, since this version of two-dimensionalism is easily and independently refutable on other grounds.
Although this is a bad result for two-dimensionalist semantic theories, it does not affect our ability to explain instances of the necessary aposteriori involving manifest kinds. No special semantics is needed because, for example, our knowledge of a truth like Water is H2O is grounded in knowledge of the molecular structure of individual water samples, which is clearly aposteriori.