George Berkeley, an Irish idealist, was a staunch nominalist and vehemently refuted the existence of abstract ideas, positions central to his philosophy, particularly his dictum "esse est percipi" (to be is to be perceived).

Berkeley's Nominalism:
Nominalism asserts that universals (general concepts like 'humanness' or 'redness') do not exist as real entities but are merely names or mental labels we apply to collections of particular things that resemble each other. For Berkeley, there are only individual, concrete particulars. When we use a general term like "man," we are simply referring to a particular man who represents other similar particular men, not to some abstract idea of 'Man' that exists independently.

Refutation of Abstract Ideas:
Berkeley launched a powerful attack on the notion of abstract ideas, which he attributed to Locke. Locke argued that we form abstract ideas by separating qualities like color or shape from particular objects, or by generalizing from particulars to form a general idea (e.g., the idea of 'triangle' that is neither equilateral, isosceles, nor scalene, nor of any specific color or size).

Berkeley declared such abstract ideas to be impossible. He argued that we cannot conceive of a triangle that has no specific properties, or of color existing apart from some colored object. Every idea in our mind, he insisted, must be a particular idea—a specific image with definite properties. What appears to be an abstract idea is simply a particular idea used to represent other similar particulars. For example, a particular red circle in my mind can represent "circle" in general, but it is still a specific, concrete circle. This refutation reinforces his nominalism and is crucial for his idealism, as it eliminates any supposed non-perceived, abstract entities that could challenge his view that reality consists solely of minds and their ideas.