Philosophers’ theories regarding the human soul? Basically there are just two of them: the first and older of the two is materialism; the second is spiritualism.
The metaphysicians who implied that matter might well have the power to think didn’t disgrace themselves as thinkers. Why not? Because they had the advantage (for in this case it is one) of expressing themselves badly. To ask whether unaided matter can think is like asking whether unaided matter can indicate the time. It’s clear already that we aren’t going to hit the rock on which Locke had the bad luck to come to grief in his speculations about whether there could be thinking matter.
The Leibnizians with their ‘monads’ have constructed an unintelligible hypothesis. Rather than materialising the soul, they spiritualised matter. How can we define a being like the so-called ‘monad’ whose nature is absolutely unknown to us?
Descartes and all the Cartesians – among whom Malebranche’s followers have long been included – went wrong in the same way, namely by dogmatising about something of which they knew nothing. They admitted two distinct substances in man, as if they had seen and counted them!