In his Meditations, Rene Descartes argues that animals are purely physical entities, having no mental or spiritual substance. Thus, Descartes concludes, animals can’t reason, think, feel pain or suffer. Animals, are mere machines with no consciousness. Use the Internet to explore the issue of animal rights. Investigate the legacy left by Rene Descartes concerning the moral status of animals. Non-human animals, on Descartes’s view, are complex organic machines, all of whose actions can be fully explained without any reference to the operation of mind in thinking.
In fact, Descartes declared, most of human behavior, like that of animals, is susceptible to simple mechanistic explanation. Cleverly designed automata could successfully mimic nearly all of what we do. Thus, Descartes argued, it is only the general ability to adapt to widely varying circumstancesand, in particular, the capacity to respond creatively in the use of languagethat provides a sure test for the presence of an immaterial soul associated with the normal human body.
But Descartes supposed that no matter how human-like an animal or machine could be made to appear in its form or operations, it would always be possible to distinguish it from a real human being by two functional criteria. Although an animal or machine may be capable of performing any one activity as well as (or even better than) we can, he argued, each human being is capable of a greater variety of different activities than could be performed by anything lacking a soul.
In a special instance of this general point, Descartes held that although an animal or machine might be made to utter sounds resembling human speech in response to specific stimuli, only an immaterial thinking substance could engage in the creative use of language required for responding appropriately to any unexpected circumstances. My puppy is a loyal companion, and my computer is a powerful instrument, but neither of them can engage in a decent conversation.
One might be surprised to learn that Descartes’ Meditations on First Philosophy, a book whose avowed purpose is to demonstrate “the existence of God and the distinction between the human soul and body,” contains so much discussion of non-human animals.  The Second, Third, Fourth, Fifth, and Sixth Objectors raise the subject of animals in order to counter Descartes’ arguments for the incorporeality of the soul, its immortality, and even the existence of God. 2] All of these Objectors advance, either directly or indirectly, the notion that animals can think in their criticisms of Descartes; his spirited replies suggest that the subject was not unimportant to him. 
Why, then, is the question of whether animals can think important for Cartesian metaphysics? The reasons will appear in starker contrast if we first examine some reasons that do not explain Descartes’ attention to the subject of animal intelligence. Descartes does not devote attention to animal intelligence out of concern that its existence would imply the falsehood of his principle of causality: i. e. the principle that all material or objective reality must proceed from some cause which contains at least as much reality either formally or eminently (the idea which undergirds Descartes’ proofs of the existence of God from his idea of Him and from the necessity of a causa sui). 
The Second Objectors raise the possibility of this principle’s falsehood by noting that animals proceed from “sun and rain and earth, which lack life”. The same animals, however, possess life, which is said to be “nobler than any merely corporeal grade of being”.  Descartes responds by simply denying the notion that “life” is nobler than inanimate being.