Locke provided an implicit criticism of Hobbes’s rejection of incorporeal substance when he noted that our idea of corporeal substance is no clearer than our idea of spiritual substance, since we take both ideas to signify the unknown “substratum” that we suppose to underlie spiritual or corporeal qualities or operations. Locke concluded that we can no more infer to the nonexistence of spirit from the lack of a clear idea of an immaterial substratum than we can infer to the nonexistence of matter from the lack of a clear idea of a material substratum.