Aristotle does not separate the formal validity of a demonstrative syllogism from its premises' degree of certainty and their aptness for explaining the conclusion. The syllogism I used earlier is not a demonstrative syllogism even though its premises and conclusion are necessarily true. In it the fact that humans are mortal is explained by humans being animals, since animals is the middle term of the syllogism. Consequently humans are not mortal merely in virtue of being animals. There is some more ultimate cause for being mortal than being an animal.
Organisms composed of different parts are subject to dissolution.
Thus all animals, including humans, are mortal because they are organized material bodies. Aristotle warns that one may be deceived in what one takes to be a demonstration because the premises are true, and produce a syllogism with a true conclusion, but the cause cited is not an immediate cause. Another kind of error occurs when a syllogism does not point out a cause at all. Aristotle illustrates it by a syllogism that concludes that the planets are near because they do not twinkle, when the fact that the planets do not twinkle is explained by their being near. Since Aristotle dedicates Posterior Analytics to demonstration, scholars concentrate on its place in Aristotle's scheme of knowledge.