If, as Moore argues, the nerve of the idealistic argument consists in the esse est percipi, Moore's paper must be considered as a final and complete refutation of idealism. But Berkeley asserts that esse is percipi only when applied to sensible things. In a similar difficulty, Plato, while looking for a meaning for existence which shall include both the corporeal and the incorporeal, offers the following definition of existence: everything which possesses any power of any kind has real existence. The objects of perception, however, being essences, do not exist in this sense. Berkeley was aware that the error contained in the assertion of the independent existence of sensible things had its root in an erroneous abstraction. Berkeley's arguments have not been universally accepted because his positive conclusions are unwarranted. He shows that what exists are always acts of perception, or perceptions; but he concludes to the existence of spiritual substance.