structural defects, dislocation, lacunae and other flaws. A perfect fluid is one that is incompressible and non-viscous — this, again, is an ideal fluid that does not exist in nature. A perfect gas is one whose molecules do not interact with each other and which have no volume of their own. Such a gas is fictitious, just as are perfectly solid, perfectly rigid, perfectly plastic and perfectly black bodies. They are termed "perfect" in the strict (non-metaphorical) sense of the word. These are all concepts that are necessary in physics, insofar as they are limiting, ideal, fictitious — insofar as they set the extreme which nature may at the most approach. In a looser sense, real things are called "perfect" if they approximate perfection more or less closely, though they be not, strictly speaking, perfect. The relation of these perfect bodies to real bodies may be illustrated by the relation of a perfect gas to a real one. The equation of state of a perfect gas is a first approximation to a quantum equation of state that results from statistical physics. Thus, the equation of state of a real gas within classical limits assumes the form of the equation of state of a perfect gas. That is, the equation of state of a perfect gas describes an ideal gas (comprising points, that is, dimensionless molecules that do not act upon one another). The perfect gas equation arose from the work of Robert Boyle, Edme Mariotte and Joseph Louis Gay-Lussac, who, in studying the properties of real gases, found formulas applicable not to these but to an ideal, perfect gas. Ethics The ethical question of perfection concerns not whether man is perfect, but whether he should be. And if he should be, then how is this to be attained? Plato seldom actually used the term "perfection", but the concept of "good", central to his philosophy, was tantamount to "perfection". He believed that approximation to the idea of perfection makes people perfect. Soon after, the Stoics introduced the concept of perfection into ethics expressly, describing it as harmony — with nature, reason, man himself. They held that such harmony—such perfection—was attainable for anyone. Plato and the Stoics had made perfection a philosophical watchword. Soon it would be transformed, in Christianity, into a religious one. The Christian doctrine of perfection is in the Gospels as well as elsewhere in the Bible. Matthew 5:48