Schleiden & Schwann were responsible for the second postulate of the Cell Theory that all living things are made of cells. Anton van Leeuwenhoek was integral to the development of the Cell Theory because he was the first to observe living cells using microscopes he had constructed. Some statements that are considered true include that some living things can exist without cells and that the ability to reproduce does not immediately mean something is a living thing. Viruses are generally considered non-living things.