People routinely ask, even in the animal rights movement, "How do you know that one claim is right, and another is wrong?" Certainly, we can have a strong intuition that a claim is right and true. But an intuition that has no other reasoning or evidence behind it is just an opinion. And moral relativism -- the idea that "we can't say what's right or wrong, only what we personally believe" -- has become a common view on the modern college campus. 

There are a few reasons, however, that we can be confident that our moral claims for animals rise beyond mere intuitions... that they are, in short, true. 

1. Independent discovery.  

The Golden Rule -- do unto others (or for others) as you would have them do unto you (or for you) -- is the foundational basis of every liberation movement. We seek to shift the oppressor's perspective and let them see, for even just a brief moment, how it feels to be in the position of the oppressed. 

But how do we know the Golden Rule is "correct"? One way is to note that it has been independently discovered over and over again, by civilizations separated by huge swaths of land or time. Peter Singer has argued that a moral escalator of reason, prevents the Golden Rule, once implemented, from being limited to a particular subclass. Over time, the circle inevitably expands. Cognitive psychologist Steven Pinker argues in a recent book that the Golden Rule may very well be a necessary element of a robust social normative system. In the language of natural selection and game theory, only systems with such a rule will survive. 

A test of truth is that it not only withstands the test of time, but spreads in the face of strong critics or contrary impulses. The spread of the Golden Rule, in the face of natural human impulses to behave otherwise, should lend credence to the notion that our moral intuitions are not just intuitions -- they are truths.  And the same can be said of the specific application of the Golden Rule to non-human animals. Activists and movements independently spring up all over the world, on behalf of the same cause and message: every animal deserves to be free. 

2. Internal consistency. External operability.

Another test for truth is internal consistency. For a claim to be true, it cannot conflict with another claim that is also true. Indeed, this is the only source of "truth" for abstract systems of knowledge such as mathematics. There is no factual validity to our most basic mathematical axioms.  And yet, some of the most prominent mathematicians in history -- figures such as Godel, Putnam, and Quine -- have confidently asserted that mathematical propositions can be true. Why? 

The first reason is that certain moral propositions may be internally consistent with other convictions, that we assume as first principles. A rule against causing suffering to another sentient-being, for example, may be an implication of a more fundamental conviction in the Golden Rule. ("If I were suffering, I would want it to stop.") So too, with a rule against discrimination against those who are weak and different. ("If I were weak or different, I would not want to be treated poorly because of that.") The truth of such propositions, in a sense, is parasitic on the more fundamental assumption. 

But that may leave us dissatisfied. Where do we get the more fundamental assumption? Here, we can also rely on an analogy to mathematics (and other abstract areas of knowledge), where we can find truth in what I call external operability , i.e. is the theory at issue necessary for having a plausible theory of morality at all, or of those things that are observable in relation to morality?  Our identification and categorization of certain principles of morality -- such as "do not kill", "treat everyone fairly", etc. -- may be arbitrary. But the platonic contours of the moral system may very well be a necessary element of any moral system, as a moral system, in the same way that certain mathematical principles may be a necessary element of any mathematical system, as a mathematical system. Similarly, while we cannot "see" moral truths, we can see that our understanding of moral behavior improves, by accepting certain moral claims as true.

3. Context Dependence. Integrated Theories. 

One common complaint about moral claims, though, is that they do not seem ironclad, in the way that physical truths are ironclad. They are context dependent. For example, we may believe "Do not kill" but make exceptions where, for example, a man is threatening harm to others. In contrast, if we say "There is a box over there", that claim does not depend on context or perception. The box exists, period.

Moral truths, then, may not appear, on first glance, to have the ironclad impregnability of factual or scientific claims. At different scales, different moral theories seem prevalent, as a factual matter, and intuitively appealing, as an emotional matter. We may believe, for example, in every sentient being's right to life, while also understanding that we cannot devote unlimited social resources to saving any particular individual's life. Is this a problem for moral "truth"?

The answer, quite simply, is "No" or, at least, "No more than any other area of knowledge." Scale or context-dependent truth, it turns out, is common in many other areas of knowledge. For example, in my field of research, behavioral economics, theories of individual choice very often do not apply when we take a broader look at the economy as a whole because systems are quite different from individuals. (Systems have "emergent" properties.) In physics, the Standard Model helps us understand the interaction of the most basic particles of the universe, on the tiniest of scales, but we must resort to a completely different (and seemingly conflicting) theory -- General Relativity -- to understand the interplay of gravity, and large objects on a universal scale. 

Context dependence, in short, does not mean that a moral claim is not true. It just means that moral claims are the products of integrated theories . Evolution cannot explain the interaction of particles. But particle physics cannot explain the evolution of life. And yet, when combined, both provide important insights into the universe. The same may be true of conventional moral theories, including the dominant divide between rights-based thinking and utilitarianism. Perhaps they are not conflicting theories. They are just integrated theories in a moral system that is context dependent. 

4. Summing up

So the next time, someone says, "That's just your opinion", don't let it slide. Our views on morality are no more "just an opinion" than our views on evolution or math. The basic moral principles that we believe in:

- have been independently discovered; 
- are internally consistent with assumptions that we all  agree to, and pragmatically necessary to maintaining a moral system that is externally operable ; and
- are part of an integrated theory of moral truth. 

And when a just idea's time has come, as the distinguished philosopher Joshua Cohen has shown, nothing in the world can stop it, from spreading across the globe.