What did Plato mean when he said that because the mind can think about immaterial objects it must be immaterial? Is this assumption true?
Descartes assumes that if it is possible to think of one thing (physical/ material) without the other (spiritual/ immaterial), then those two things are not identical. Is this assumption true?
Do definitions of philosophy assume that humans have certain abilities? Do they assume that we can understand ourselves and the universe?
Answer by Jürgen Lawrenz
Ask yourself: if the mind was a physical thing, how could it discern something immaterial (e.g. love, justice, beauty, God)? None of these can be touched, seen, heard or tasted, and therefore they would be inaccessible to such a mind. Further such a mind would only be able to process factual information and relay them to experience, and from there to language. But metaphysical states (like being in love, having a religious ecstasy) cannot be experienced, for the same reason again that our senses (or instruments) don’t participate and/or discern. In that sense, Plato’s proposition is undoubtedly true.
As for Descartes, the idea is obviously the same, even though it is couched in different words. Consider yourself sensing a physical object: Now your mind, in order to perceive this object, must pick up something from it which is not material, i.e. colour (we call this the phenomenon of the object). If you cannot do this, then there is no such object. And it works the other way around just as well. You cannot discern a phenomenon without there being a physical correlate in the actual world. This argument, by the way, killed the belief in ghosts, witches, sorcerers etc., which are all based in the erroneous idea that you can discern phenomena without some physical existent causing them.
The answer to your last question is a qualified ‘yes’. A definition is in fact necessary for us to do philosophy, science, technology (think of medicine without definitions: how could you find a cure?). Definitions include every relevant feature of a group of objects or creatures, which helps us to identify what they are when we encounter a particular specimen. This ability was pretty much proved by biologist Ernst Mayr, who found that natives in New Guinea, without any kind of science whatever, necessarily developed a taxonomy of the wildlife in their habitat. It was a completely spontaneous classification and corresponded almost 100% to his own (scientific) research over 3-4 years.
The qualification I mentioned works this way: We must assume that definitions help us to survive, gather accurate information and assist us in our understanding of he uses of natural products and processes for our own benefit. To that extent we assume that humans are capable of understanding both the world and ourselves. The rider is ‘to a limited and practical extent’. When we go beyond the scope of practical needs and empirical conditions, we put ourselves adrift in the realm of speculation. I’m reasoning here in a perfectly cold and unattached manner: whatever we speculate about, whether in philosophy, religion or science, may be true or false, but it cannot be certain except to the extent that a portion of it may became practically usable or empirically testable (by experiment). What this means is that hundreds of millions of people in history have believed themselves to be absolutely sure that they had certain knowledge (e.g. of their religious faith), yet in a detached, objective perspectives none of these beliefs could ever have been proved, they never have been proved, and unless Descartes is wrong, never will be proved. This is because these issues or existents have no physical correlate. And now we’re back with Plato.