 On a bottom-up approach, on the other hand, the double exclusion plays a comparatively minor role, and thus some of the Kumārila-Uddyotakara criticisms may be less telling. Causal chains and error are what serve to bridge the scheme-content gap, rather than the descriptive content of the apoha. It is notable that such reliance on causality seems to be entirely absent in Dignāga: it is Dharmakīrti’s major contribution to the theory. The way words link to things is thus primarily explained through the existence of a causal chain from particular things to perceptions to thoughts and to the utterances of words—in short we have a type of causal theory of  reference.[40]  While modern day causal theories of reference—such as that of Saul Kripke—are typically used to explain how proper names can refer without descriptive content, the theory can be extended in various ways to kind terms. Such is the move of Hilary Putnam who emphasized that descriptive content, or at its simplest just “what is in one’s head,” is not enough to distinguish between seemingly similar kinds. Dharmakīrti’s causal theory too is not restricted to the reference of proper names, but also seeks to explain causally reference to kinds of things. The causal connection is needed to guarantee “contact” with the world, i.e., that language ultimately must rest upon perception, which is non-abitrary in that it is “fixed” by things. Dharmakīrti thus explains how an individual (to be called “Jones”) can think about and refer to such and such an object in the world through a complex and long causal chain from the particular objects to the mental invention of a quasi-universal and the use of a word on a specific occasion. Thoughts and talk of blue are thus about blue things because only blue things play the appropriate causal role in leading to the thought and finally the  word.[41]