Cartesians had an idea about animals. They believed that animals were machines without minds. Here’s the story:
They believed in the mechanical philosophy. Everything could be explained in mechanical terms. But Descartes (being the genius that he was) felt that human creativity, the highest embodiment of which he considered the human language, could NOT be explained in mechanical terms. That led him and his followers to come up with the (at that time) scientific hypothesis of mind body duality. There is the body, which is basically everything in this world, and then there is the mind, which is human creativity.
Now Descartes noticed that animals could be trained to perform different tasks. At the same time, many Cartesians were building amazing robots (automatons to be exact) which could do all kinds of tasks. But Descartes noticed that no matter what, animals simply couldn’t be trained to have a language!
So Cartesians leapt to the straightforward, scientific (according to the science of the day) conclusion that animals were machines with no minds. See the connection?
It all fits in doesn’t it? History of science is truly fascinating!!
(Endnote: Mind-body dualism has rightly been debunked, even since Newton destroyed the mechanical philosophy)