William of Ockham was one of the most prominent philosophers of medieval SCHOLASTICISM . He is known today principally for a maxim we call "Ockham's razor": Entities are not to be multiplied without necessity. These aren't his own words, but he came up with the general idea which he expressed more obscurely: "It is vain to do with more what can be done with fewer." ["Frustra fit per flura quod fieri potest per pauciora."] In yet other words, if you can explain something without postulating some unnecessary theoretical entity, then do so. Perhaps you can see how such an idea not only goes against Platonic idealism, but also contains a "dangerous" suggestion of atheism!
A more contemporary generalization of Ockham's razor is the "KISS" principle: Keep It Simple, Stupid!