What do I mean by “primary philosophy”?
Primary philosophy is the branch or branches of philosophy which provide the necessary foundational principles from which all secondary (or speculative) philosophy must follow.
I intentionally avoid the phrase “foundational philosophy” simply to avoid the foundationalist v. coherentist debate, and I will not here directly defend my position that all true philosophy must be both foundational and coherent.
Also, I would like to distinguish primary philosophy from “first philosophy,” insofar as the latter may be understood as implying a temporal order. For me, and perhaps for many others, my “first” branch of philosophy was normative ethics. I would debate on such issues as war and peace, capital punishment, abortion, etc., but it seemed that very little progress was being made. This result, in addition to my own skeptical doubts concerning such matters, led me to realize the necessity for the foundational principles provided by primary philosophy.
What branch or branches of philosophy are rightly included in primary philosophy?
I can certainly imagine a deeply religious thinker who may claim that theology is primary; he or she may argue that God is the origin of everything and, therefore, God alone provides the foundational principles for philosophy. And perhaps some materialists may argue that primary philosophy consists of empirical physics. However, I do not take these views very seriously, and I suspect that the most popular candidates for primary philosophy are epistemology and ontology.
Those who prefer epistemology may assert that all philosophy presumes knowledge and, therefore, all philosophy must follow from principles concerning the nature and scope of knowledge. Likewise, those who argue for ontology may insist that all philosophy presumes existence and, thus, must follow from ontological principles. To the extent that these views do not conflict, I agree with both.
My view is that primary philosophy does not consist of a single branch, but rather, four branches: logic, ontology, phenomenology, and epistemology. That is, all true philosophy consists of and moreover requires logical consistency, existence, conscious experience, and self-conscious experience, i.e., knowledge.
Though I count each of these four branches as primary, I will also claim that some are more fundamental than others. It seems that logic is the most fundamental of the primary branches, with non-contradiction as the most fundamental principle. However, logic alone cannot constitute the whole of primary philosophy, for non-contradiction by itself does not provide a foundation for speculative philosophy. For instance, according to pure logic, it is possible that there is no existence, no conscious experience, nor self-conscious experience (without which there can be no philosophy). Logic merely dictates that there must be either existence or no existence, and not both, etc. For similar reasons, on which I will not go into detail, I recognize ontology more fundamental than phenomenology, and phenomenology as more fundamental than epistemology. Nevertheless, each is primary, for each is implicit in all true philosophical thought.
What branches are to be considered secondary philosophy?
The branches of philosophy I consider to be secondary (or speculative) include all those which are not primary: metaphysics, physics, aesthetics, ethics, politics, theology, etc. Unlike primary philosophy, these branches are not necessary for philosophical thought, nor are their respective objects of study self-evident. That is, in doing metaphysics, one assumes existence external to conscious experience and speculates on its nature by applying the principles of primary philosophy. Likewise, physics assumes physical existence, reliability of the senses, and inductive reasoning; ethics assumes free-will and objective standard for decision making, and so on.
A final note:
I realize that this topic is broad in scope and brief in delivery. I predict and look forward to much disagreement.