The fourth proof is very much like the first, but is based upon the ideas of potentiality and actuality instead of motion. But when we consider that Aristotle defines motion in terms of potentiality and actuality, the fourth proof is identical with the first. It reads in Maimonides as follows: We see constantly things existing potentially and coming into actuality. Every such thing must have an agent outside (18). It is clear, too, that this agent was first an agent potentially and then became one actually. This potentiality was due either to an obstacle in the agent himself or to the absence of a certain relation between the agent and its effect. In order that the potential agent may become an actual agent, there is need of another agent to remove the obstacle or to bring about the needed relation between the agent and the thing to be acted upon. This agent requires another agent, and so it goes _ad infinitum_. As this is impossible, we must stop somewhere with an agent that is always actual and in one condition. This agent cannot be material, but must be a "separate" (24). But the _separate_ in which there is no kind of potentiality and which exists _per se_, is God. As we have already proved him incorporeal, he is one (16).[274] We must now analyze the expressions _incorporeal_ and _one_, and see what in strictness they imply, and how our logical deductions agree with Scripture. Many persons, misled by the metaphorical expressions in the Bible, think of God as having a body with organs and senses on the analogy of ours. Others are not so crude as to think of God in anthropomorphic terms, nor are they polytheists, and yet for the same reason, namely, misunderstanding of Scriptural expressions, ascribe a plurality of essential attributes to God. We must therefore insist on the absolute incorporeality of God and explain the purpose of Scripture in expressing itself in anthropomorphic terms, and on the other hand emphasize the absolute unity of God against the believers in essential attributes.