I hate Plato.
He by himself set back science at least a thousand years, maybe two thousand.
Here is how he did it: he said all matter was the same stuff, and only the "spirit" of matter changed to give this prima materia its properties. With this idea he could propose the transformation of substances into other substances (for example, he said a fire converted "earth" (the solid wood) into air (the hot exhalation of a fire) and a different earth (the smoke) and a third earth (ash). Idiot. Why couldn't the doofus have just paid attention to what he saw? Or the water cycle: water became air through evaporation over a fire (fire + water = air, what a nincompoop), then as it cools it becomes water again (duh!). Why the heck could he not just admit that it remained water the entire time.
From this came alchemy, pretty much a waste of 1400 years of good brains. And Galen's four humors. And hundreds of thousands of patients killed by well-meaning but ill-informed teachers.