Prescriptivism is the non-cognitivist meta-ethical view that moral statements are intended as commands or imperatives by the person making the statement for others to act or refrain from acting in a particular way. R.M. Hare is the most famous proponent of prescriptivism. He called his system universal prescriptivism, which means that the person making a moral judgment prescribing how to act or not to act is committed to the same judgment in all situations with similar facts. For example, the statement “suicide is wrong” is understood to mean “don’t commit suicide,” and is universal and thus understood to apply in all situations so that all people are commanded not to commit suicide by a moral statement indicating that suicide is wrong.
While it makes sense to view moral statements as commands or prescriptions of how others ought to act, as, speaking from my limited personal experience, most moral judgments seem to include the implication that people ought to act consistently with that moral judgment, there are two major problems with universal prescriptivism.
The first is a problem with prescriptivism of any kind, namely that most moral statements made by most people most of the time seem to refer, whether implicitly or explicitly, to purported facts of the universe upon which valid moral judgments are believed to be ultimately based, whether those facts are understood to be empirically observable natural properties or are understood to be independent of nature and accessible only to intuition or pure reason. In other words, the same critique previously made of emotivism and other forms of non-cognitivism applies equally to prescriptivism, whether understood as universal or not. On the other hand, if people intend moral statements to constitute imperatives or commands of how to act or not act, then it makes sense to understand such statements as imperatives.
The second is a problem that applies specifically to universal prescriptivism, namely that there is no basis for believing that a person must or ought to be committed to the same moral judgment in every situation. It is not self-evident that hypocrisy or moral inconsistency is wrong. To hold that hypocrisy or inconsistency is wrong without establishing this point through rational argumentation is to assume the truth of the premise (that hypocrisy or inconsistency is wrong) without establishing the truth of that premise through rational argument, and is thus a form of begging the question.
The second problem invalidates universal prescriptivism just as it invalidates many other universalist theories of ethics, but the first problem only invalidates prescriptivism as a meta-ethical theory when the person making the particular moral statement in question intends that statement to refer to a fact or facts of the universe knowable either empirically as properties of nature or through intuition or a priori reason as properties of the universe independent of natural laws.
As with emotivism, while admittedly most people making moral statements intend them to refer to factual states of affairs, it is possible for a person making a moral statement to intend that statement as a pure prescription or imperative independent of any factual state of affairs. When a moral statement is intended in this latter way, prescriptivism is a valid meta-ethical theory, but not otherwise.