text: The formal cause is its form, i.e., the arrangement of that matter.It tells one what a thing is, that a thing is determined by the definition, form, pattern, essence, whole, synthesis or archetype.It embraces the account of causes in terms of fundamental principles or general laws, as the whole (i.e., macrostructure) is the cause of its parts, a relationship known as the whole-part causation.Plainly put, the formal cause is the idea in the mind of the sculptor that brings the sculpture into being.A simple example of the formal cause is the mental image or idea that allows an artist, architect, or engineer to create a drawing.The efficient cause is "the primary source", or that from which the change under consideration proceeds.It identifies 'what makes of what is made and what causes change of what is changed' and so suggests all sorts of agents, non-living or living, acting as the sources of change or movement or rest.Representing the current understanding of causality as the relation of cause and effect, this covers the modern definitions of "cause" as either the agent or agency or particular events or states of affairs.In the case of two dominoes, when the first is knocked over it causes the second also to fall over.In the case of animals, this agency is a combination of how it develops from the egg, and how its body functions.The final cause (telos) is its purpose, the reason why a thing exists or is done, including both purposeful and instrumental actions and activities.The final cause is the purpose or function that something is supposed to serve.This covers modern ideas of motivating causes, such as volition.In the case of living things, it implies adaptation to a particular way of life.Optics Aristotle describes experiments in optics using a camera obscura in Problems, book 15.The apparatus consisted of a dark chamber with a small aperture that let light in.