What is God; Via Negativa

I find the concept of via negatativa to be unusual. The definition of anything cannot be what it is not or it is in fact not a definition. To define something it must be clearly outlined. To say that we only know what God is not is not knowing what God is. And furthermore, how are we certain of what God is not? Is there some sort of proof for the notion that God is simple rather than complex? Are these not merely the speculations of men? Via Negative is not a solid argument for proveing exactly what God is.