"Descartes reasoned that since animals are not rational, they are not conscious, and since they are not conscious, they cannot even be aware of pain; their piteous howls during the horrible experiments he conducted on them were to him mere reflex, the unfelt expression of material reactions akin to the shrieking of a teakettle."
The Cartesian world view on biology has been catastrophic to our fellow mammals and even today aspects of this perspective survives. Physiologically, humans are not very different from other mammals, even if we want to be. We all have a limbic system and that means we all have the same emotions. Animals have feelings and emotions. When things happen to them, they react with emotions be it sorrow, anger, happiness, fear.
The cartesian view on biology was a necessity to avoid public burning, and essential to make positive science possible.
Before Descartes, it wasn't at all clear and obvious that mere things had no intention, no wants, no "soul". In fact, Aristotelian physics considered that a stone drops because of its "grave" nature, that it somehow wishes to go down (The last gasp of this magical thinking lasted well into the 19th century, until Pasteur finally demonstrated that there is no spontaneous generation).
By excluding everything non-human from any "soul", Descartes made science as we know it possible. It was a paradigm shift of immense conceptual importance.
The Cartesian world view on biology has been catastrophic to our fellow mammals and even today aspects of this perspective survives. Physiologically, humans are not very different from other mammals, even if we want to be. We all have a limbic system and that means we all have the same emotions. Animals have feelings and emotions. When things happen to them, they react with emotions be it sorrow, anger, happiness, fear.