This paper opens by distinguishing between the multiple concepts of system and the philosophical idea of system. It then goes on to discuss the differences between systems and other proximate ideas, such as whole, set, aggregate and structure. Subsequently, it proposes a definition of system, and then lays out three classifications of systems. When elaborating a general definition of system, the main challenge is finding a general criterion with which to characterize both technical systems built by men (for instance, machines), and scientific systems independent of human subjects (for instance, the solar system). The criterion proposed in the text solves this difficulty. Lastly, from the tenets of Gustavo Bueno’s hyperrealist philosophy of categorial closure, I put forward the consideration of the sciences as systems.