The Principle of Common Essence (PCE) is based on the fact (an actual experimental finding) that humans, unlike most other animals except orangutans, perceive that other individuals of their species have minds and inner experiences like their own. This has been called “a theory of mind”.

I argue that for humans this is the basis of morality, ethical systems, and conscience (the moral sense). However, it can also be the basis of the formation of enemy images, by the psychological principle of projection. In other words, the PCE and the theory of mind can be the origin of both Good and Evil: the knowledge of other minds is what came, mythically, from eating the fruit of the tree of knowledge in Eden. It is both our glory and possibly our fatal flaw.

Walk past a store window and see a dressed figure there. First you think that it is a mannequin, but then it moves, and you perceive a person. What a difference that makes to the perception! Hear a recorded message on the telephone, and then a real voice comes in. Again, what a fundamental switch in perception! We might see the person as a friend or a foe, but never as a thing. This is the difference between Martin Buber’s “Thou” and “It”.

So what consequences does this have for our systems of morality? How ought we to behave toward these perceived, unique, non-fungible persons? I will consider here only the positive side, how we ought to behave, not the negative evil side.

First, since we ourselves do not want to be harmed (I exclude suicidal persons), we should avoid harming others. “Do no harm” is the first principle of medical ethics in the Hippocratic Oath, and should be the first principle of behaviour by any other individual. It is Gandhi’s “ahimsa”. However, it is not clear-cut.

We SHOULD shoot down the crazy pilot out to drop Hbombs on an enemy who is capable of retaliation (or even not capable), because of the much greater harm that would come from the accomplishment of his self-assigned mission.

Some kind of rational moral calculus enters here, a consequential ethic. This can slide into the utilitarian principle of aiming at “the greatest happiness of the greatest number”, but again caution is required here before completing the slide, as pointed out in the discussion of the second principle derived from the PCE.

This second principle states that, because our own mind strives for happiness, therefore we should strive to increase the happiness of the others who are so like us. This is the positive side of not doing harm.

However, should we agree that it is moral to kill a limited small number of human beings in order to increase the happiness of many other human beings? There are several difficulties: 1. It may not be possible to quantify “happiness” (utility). 2. The utilities of different individuals cannot be simply added; they differ in quality. 3. Even if we ignore the first two points, and if the sum of utilities would then come out positive, and thus conform to the utilitarian principle of “the greatest happiness of the greatest number”, our moral sense feels uncomfortable about totally sacrificing some individuals for a marginal increase of happiness of the many others. Perhaps the many others are just racially prejudiced against some minority and want to see them eliminated, in order to feel happier.

The term “marginal increase in happiness” pinpoints the difficulty. On a semi-quantitative utility scale, death or slow torture is zero or negative, while happiness at the upper end yields diminishing returns, like giving one extra dollar to a millionnaire. Maybe there is even an inflection point (sharp change in slope) at some point, as in Alcock’s life expectancy vs. GNP per capita curve for nations.

Perhaps this could be operationalized by using the product of individual utilities rather than the sum. Then any individual’s zero makes the product zero (or anyone’s negative utility makes the product negative*), while the sum would be hardly affected.

However, there is still an exception. If not killing the crazy pilot about to drop H-bombs will kill millions of people (even more if retaliation is triggered), then our moral sense indicates that he should be killed. It is as if the zeros add up, unlike in mathematics.

PCE moral calculations are not purely utilitarian, nor do they totally exclude evil means, as in n… ethics. Moreover, the calculations are often imprecise, espacially if probabilities rather than certainties of outcome are involved, or even more so if the uncertainty is so profound that estimates of probability cannot be made at all.

Do we have to conclude that the human moral sense based on PCE is inborn or intuitive, as the term “conscience” implies? Is conscience, like consciousness, an emergent quality of a complex brain? Is it then not truly amenable to ordinary rational moral calculations?

What about extensions of PCE to non-human creatures? This would involve discounting, again an imprecise, nonquantifiable notion. Plants count less than animals, insects less than mammals, bacteria the least, unless useful to humans. This is anthropocentric, but perhaps unavoidable. Perhaps the PCE must pay attention to a scaling down of the common essence in other creatures. This then depends on comparisons with the HUMAN common essence, taken as the standard. Are we justified in doing this, just because of who we are? This is no absolute measure of value, but this is the PCE we are born with. We are not capable of a superior, God-like perspective.

The extended PCE which we have been considering is summed up as “respect for life”. It can never be absolute, as the previous discussion shows. Moreover, we need to kill some of the members of the living world in order to eat and to defend ourselves and our crops against diseases. That it should be done with an “apology”, as native hunters do, seems hypocritical. The prey surely never give permission. The commandment “thou shall not kill” in the Old Testament was meant to apply only within the Hebrew community, as the rest of the Scriptures makes clear. Empathy then did not extend even to all humans.

So what is moral conscience? It seems to operate intuitively, through the emotion of empathy, based on a theory of mind in other persons or even other living beings. It is not rational; by this I do not mean that it is irrational, but only that it cannot be entirely encompassed by quantitative reasoning, although partial attempts can be made, as was done here. However, the reader will note that when the moral calculations came to an impasse, I had recourse to the intuitive moral sense.

Conscience can be ignored, but most humans when acting against it do know it, are aware of doing evil. There are a few defective humans who lack this sense of guilt; they are called sociopaths. In minor matters, many people put themselves first, discounting neighbours and especially enemies. The New Testament warns against this, urging us to “love your neighbour as yourself” (i.e. equally, without discounting), and even to love our enemies and return good for evil (turn the other cheek). We can conclude that the NT version of the PCE (Christian ethics) urges us never to discount other human beings. It is to be fervently wished that nominal Christians would apply this in practice.

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• This does not work, because if an even number of individuals are killed, the minuses would cancel out. Better stick with the zero.