Final answer:
Descartes' theory posits animals as non-reasoning automata, devoid of mental states. An objection could point to empirical evidence of animals displaying pain and learning behaviors, backed by neuroscientific findings, that imply the presence of mental states in animals.
Step-by-step explanation:
The Cartesian theory of animal nature posits that animals are automata without souls or mental states, thus lacking the ability to reason, unlike humans. However, a plausible objection to this theory could be that the observable behavior and indicators of distress in animals suggest they do experience sensations such as pain, which contradicts the Cartesian view that they have no mental states at all. If animals didn't have mental states, we wouldn't expect them to show preferences, learn from experience, or exhibit behaviors consistent with suffering, all of which have been observed in various animal species. Moreover, recent scientific studies in cognitive ethology and neuroscience have shown that animals do possess complex nervous systems capable of processing pain and emotions, further challenging Descartes' assertion.