Any reader of The Capitalist Manifesto should be familiar with the three principles of economic justice developed by Louis Kelso and Mortimer Adler. They are explained in Chapter 5. As analyzed by Kelso and Adler, the principles of economic justice are firmly rooted in the precepts of the natural law as understood within an Aristotelian framework. They are therefore fully compatible with (for example), the social doctrine of the Catholic Church based on the thought of Thomas Aquinas, the social teachings of Judaism from the perspective of Moses Maimonides, and of Islam within the school established by Ibn Khaldûn.
Consistent, yes, but to what degree are the principles themselves either implied or explicitly stated in, say, the social teachings of the Catholic Church, especially since Rerum Novarum was issued in 1891? Other than to cite the analysis of capitalism by Leo XIII and Pius XI, Adler doesn't make direct references to Catholic social teaching. Instead, he made it clear that the principles in The Capitalist Manifesto are consistent with the natural law on which Catholic social teaching — and that of Judaism and Islam — is based.
Unfortunately, far too many people today are unable to "connect the dots." Unless a truth is stated explicitly to them (and then only if it does not contradict an opinion they hold), they seem incapable of grasping the obvious. Further, as Adler pointed out in Ten Philosophical Mistakes (1985), many people have rendered themselves unable to distinguish between knowledge (that which is manifestly or self-evidently true and can be verified), and opinion (that which we hold because we believe it, although it lacks the absolute certitude of knowledge).
Definition is sometimes an important part of knowledge. If we (using Adler's example) define a triangle as a three-sided plane figure, then any three-sided plane figure is necessarily a triangle. Further, we cannot (at least honestly) decide that, while you may continue to languish in ignorance and call a three-sided plane figure a triangle, we in our advanced state of being are going to call a three-sided plane figure a rectangle, and a four-sided plane figure a triangle. By doing so we effectively "abolish" both triangles and rectangles, for we, like Humpty Dumpty in Through the Looking Glass, have made their definitions — and thus the terms — meaningless.
Opinion and knowledge are not in any way inferior to one another. They are different ways of apprehending reality. A true opinion is just as true — and is true in the same way — as knowledge. There are not different kinds of truth, even though different types of knowing apply to different areas of human thought. The difference is that an opinion is not true knowing, but one form of apprehension, because an opinion is not, and cannot be, necessarily true.
Using definition is another way of saying that whether something is knowledge or opinion sometimes rests on the principles or assumptions we make. Thus we can say (somewhat tautologically) that if we define a triangle as a three-sided plane figure, then a three-sided plane figure is necessarily a triangle.
The corollary to using definition in knowing is that if we reject a certain definition, we cannot justly or fairly (or even intelligently) judge the conclusions or position of those who continue to adhere to the old definition, regardless how right we think we are in changing the definition. As G. K. Chesterton observed in The Dumb Ox, his short biographical sketch of St. Thomas Aquinas, we can always prove somebody else wrong on our principles, including our definitions. The key to honest debate is to prove the other wrong on his principles or definitions.
Consequently, no one can say (for example) that a Catholic's belief in the Real Presence (the belief that Christ is physically present in the Eucharist) renders a Muslim's proof of a mathematical theorem invalid, or a Jew's economic analysis false. Nor does a Catholic's belief in the teachings of his or her faith render his or her scientific theories valid or invalid. These are different systems, usually called "faith" and "reason," employing different principles, assumptions and definitions. The fact that the principles differ between the two systems does not mean that the one is false in terms of the other. All it proves is that they are different in terms of the other, and cannot be judged by the same principles.