The Significance of Sources
People often ask if there is a source in Judaism for a given idea, and whether it's in Chazal, a Rishon, or an Acharon. But they frequently don't seem to have given much thought to the nature of the significance of finding sources.
Allow me to explain. There are some legitimate reasons for wanting to know if there is a source for something, and what the antiquity of that source is. As a traditional, religious system, the antiquity of a source is of great significance for its legal authority. The opinion of a Tanna carries greater weight than that of an Amora; the view of an Amora carries greater weight than that of a Rishon; that of a Rishon is of greater weight than the view of an Acharon.
On the other hand, finding an ancient source for something does not necessarily mean that it is of practical halachic significance today. One must also take into account how Judaism develops over time. For example, there is a solitary opinion in the Mishnah that one may make a shofar from a cow's horn. However, since this has been summarily rejected by every authority since then, it is no longer of halachic significance. And elsewhere we have discussed how Talmudic warnings about eating peeled onions were not incorporated into halachah and should not be revived.
Finding a source for a view is also significant for non-halachic matters, insofar as evaluating the extent to which it represents classical, traditional Judaism. For example, while it's hard to conclusively point to any reference to the afterlife in Tanach, there are certainly abundant mentions of it in the Talmud. On the other hand, reincarnation has no mention in either Tanach or Talmud, and some of the earliest sources in the Geonim and Rishonim to discuss it actually reject it.
But the mistake that many people seem to make is to believe that finding a source for something gives it factual physical or metaphysical reality. It doesn't. You can find sources, even ancient sources, for all kinds of weird and wonderful things - from geese growing on trees to Binyamin being a werewolf. Chazal themselves believed in demons, and this did not prevent Rambam and others from rejecting their existence. Sources might define Judaism, but they don't define reality.