Plato has a few artuments to try to demonstrate the nature of the mind. One of which, in the Phaedo appeals to the fact that we find from one state, it follows a contrary state; or to put it bluntly, that an opposite begets an opposite. After youth comes aging, after light comes day and after life comes death.
The appeal to the immortality of the soul is that like night and day, we find a repetition once a cycle is completed. A day is followed by a night, and a day follows that. It striked me that some people have a similar belief about the original homeopathic programme during the 18tC. What is to note in both notions of the eternity of the soul and homeopathy is that both beliefs are an appeal to a metaphysical thesis.
While metaphysics has a place in our beliefs and understanding about the world; it is far from this kind of direct appeal. The issues of metaphysics rather, emerge from our conduct of empirical studies. Once one has appropriated research about the natural world, metaphysical theories seem better placed to appropriate said research: do theoretical entities exist? for instance. Is there a unity in the body of knowledge of science?
It is a deceptively simple notion that opposites beget opposites, and often the arguement concerning the existence of the soul drag us to very thorny metaphysical territory. The applicability of such a claim of opposites begetting opposites seems far to vague to constitute a thesis. Does one make a claim about the eternal reoccurence from Plato’s claim? Is one to justify vaccinations? Or is one to justify the rationality of their belief in the earth’s rotation on that basis. This simplistic belief seems prima facie correct.
I’m not sure how knowledge of antibodies (in the vaccination case) or universal gravitation apply. It is said that the simplest explanation is the best one; but simple in this sense is different to naive.