In this thesis, I shall explore the theoretical and empirical expositions regarding the causal mechanisms of cognitive growth. I shall do this in order to determine if biological epistemic theories of cognitive systems can be justified. It will be necessary in this thesis for me to adopt a multidisciplinary stance from Philosophy and Psychology. It will try to investigate from these two perspectives what it means to be a cognitive creature. However, I shall argue, if taken singularly, each standpoint fails to provide an adequate account of cognition that is necessarily based on adaptive, evolutionary constructs. During this thesis I will primarily focus on the major arguments in Philosophy that show a tight coupling between language, cognition and rationality. More specifically I will examine in detail Donald Davidson’s holistic account of what it is to be a rational, cognitive creature. I will show in the thesis, through comparative experimental evidence, that the causal mechanisms of cognitive growth, and thus thought may not be language. Consequently, Philosophical arguments that are based on tight relationships of thought and language will not be able to deliver a true account of cognition. I will demonstrate that Davidson’s philosophy has suffered from not being able to ground his philosophical perspectives on the relationship of language, cognition and rationality within an empirical programme and consequently it makes fundamental errors. Davidson’s account does not take on board the recent (and not so recent) empirical based work on primates which show the possible mechanisms of cognitive growth, which are independent of language. Similarly, I will also show that Psychology, which does provide us with the means to deliver an empirical account of cognition, due to its history based on Behaviourism, does not have the right causal mechanisms nor language to talk about the nature of complex cognition. I will show how Associationistic Psychology mischaracterises what it is to be cognitive and consequently, like philosophy, cannot deliver an accurate ontology of cognition. I intend in this thesis to provide a bridge between the two schools by adopting a comparative psychological approach. By using this comparative perspective, a more accurate theory of cognition may be possible and one that is not contaminated by language or any other cultural symbolic systems. I aim by the end of the thesis to be in a position which will hopefully allow modification of Davidson’s condition on possessing beliefs, a creature must have beliefs about beliefs. This modification will be based on an evolutionary account of what may or may not eventually turn out to be the precursors of higher cognitive states.