What guide Supreme Court decisions?

Many would say the simple answer, which is "the Constitution". That is not, however, the most correct answer. What guides Supreme Court decisions are firstly the politics and socio-cultural experience of the individual Supreme Court Justice writing the opinions, which is the expression of the Court's decision. These are expressed in the choice by the executive branch in the form of the President, and confirmed by the Senate, of which jurists to appoint to the Court, based upon the President's perception of the liberality or conservatism of the jurist's political viewpoints.

Secondly, what guides Supreme Court decisions are the legal principles expressed as stare decisis, which is law Latin, literally "that which is decided". This term refers to the sum total of the applicable legal precepts and interpretations derived from over two hundred years of U.S. Constitutional common law.

These are precepts such as "substantive and procedural due process of law"; "separation of powers between the three branches of government"; "equal protection of the laws to be accorded to similarly-situated persons and groups"; the "supremacy clause"; the "right to remain silent"; the "right to confront the witnesses against one", to name only a very few.

That is, every Supreme Court decision does not reinvent the wheel by deriving all over again long-established legal principles, their nuances, and past applications, from the text of the Constitution, and based upon the perception of the intent of its drafters and ratifiers. This is not say that with the passage of time, that some of these principles fall into disuse or are abrogated in favor of new interpretations, because over the decades this is, in fact, what has indeed characterized the U.S. Constitution as a "living document", and the resultant Constitutional common law.

Thus, what is considered and cited comes directly from case law, or "common law"--judge-made law--which is derived from the provisions set forth in the seven articles of, and the twenty-seven amendments to, the United States Constitution.