Pre-Socratic reaction to this deficit was varied. As substance theorists they accepted a priori the hypothesis that appearances are deceiving, that reality is to be reached through reasoning. Parmenides reasoned that if everything is identical to being and being is a category of the same thing then there can be neither differences between things nor any change. To be different, or to change, would amount to becoming or being non-being; that is, not existing. Therefore, being is a homogeneous and non-differentiated sphere and the appearance of beings is illusory. Heraclitus, on the other hand, foreshadowed modern thought by denying existence. Reality does not exist, it flows, and beings are an illusion upon the flow.