Individual Opinion, it is sometimes suggested, accounts for the existence of moral truths. This view, called ethical subjectivism, argues that moral truths are determined by individuals. On this theory, if you believe the willful taking of innocent life is wrong, then it is wrong for you. On the other hand, if I do not believe the willful taking of innocent life is wrong, then it is not wrong for me. Surely, we can all see what is wrong with this view. This view denies that objective moral truths exist. On this view, moral truths are not objective but subjective. But, as noted in previous blogs, moral truths must be objective—that is, some things must always be wrong and others always right regardless of what anybody thinks about them. This reason alone is enough to reject ethical subjectivism as the right account of the moral realm. But, there are many more problems with ethical subjectivism which should be considered.
First, Ethical Subjectivism violates the law of non-contradiction which says regarding propositions that a truth claim cannot be both true and false in the same way at the same time. So, for example, it cannot be true both that a woman is pregnant and is not pregnant. She is either one or the other. To claim to be both, would be to assert contradictory claims. In such a case, both claims cannot be true. But, on ethical subjectivism, individuals hold conflicting moral claims constantly, and the theory treats them as true. As in the example above, it cannot be the case both that murder is wrong and that murder is not wrong. But, on ethical subjectivism such a situation is possible. For this reason, we can conclude via reductio ad absurdum that ethical subjectivism is false.
In addition to this problem, ethical subjectivism also suffers from another difficulty—if ethical subjectivism is true, then no person is ever wrong because each decides for himself or herself what is right and wrong. But, surely this can't be right. For every one of us is sure that he or she has been wronged by someone else in the course of life. Thinking this way, though, doesn't fit with ethical subjectivism.
Also, on ethical subjectivism, no normative laws can be made because individuals have the right to decide for themselves what is right and what is wrong and no one else can decide this for them. But, because we believe objective moral truths exist, we believe we have the right to expect all individuals to live by certain moral rules. This view simply won't do on ethical subjectivism.
Finally, when two people fight about what is right or wrong, they are implying that they believe there is a moral law outside of themselves which governs them—they are essentially acknowledging that they do not have the right in themselves to determine right or wrong for themselves. But, if ethical subjectivism is true, all such quarreling makes no sense.
Surely, it is obvious that ethical subjectivism not only fails to account for the existence of objective moral truths but that it also simply fails to describe accurately the moral realm as we experience it. For these reasons, we can rule out ethical subjectivism as the explanation for the existence of objective moral truths.
 “Ethics - Introduction to Ethics: Subjectivism.” BBC, www.bbc.co.uk/ethics/introduction/subjectivism.shtml.
 Slick, Matt. “Christian Apologetics & Research Ministry.” CARM.org, CARM, 10 May 2017, carm.org/dictionary-law-of-non-contradiction.
 Gert, Bernard and Gert, Joshua, "The Definition of Morality", The Stanford Encyclopedia of Philosophy (Spring 2016 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/arc...;.