There has been much debate about whether different groups of people diverge in their conceptual schemes (according to some criterion of identity for conceptual schemes). But it has not been questioned that each group has a single conceptual scheme, still less that an individual has one and only one conceptual scheme. It is assumed that we each possess a single conceptual scheme, even if it differs from the scheme of others. That is not self-evident, however: couldn’t a single individual possess several conceptual schemes, more or less harmonious with each other? What about the idea that the conscious mind and the unconscious mind differ in their conceptual schemes? What about a schizoid individual? Could the right hemisphere and the left hemisphere differ in their basic outlook on the world? What if the human brain still housed the conceptual apparatus of our ancestors, going back to fish? What if you just happened to think about the world in radically different ways on alternate days because of some divine mischief?
I am going to suggest something less exotic, indeed on reflection quite familiar, namely: we already have three commonsense conceptual schemes that fit uneasily together—three very different ways of conceiving the world that coexist in our minds. I will call these the Referential Conceptual Scheme, the Epistemic Conceptual Scheme, and the Ontological Conceptual Scheme (for convenience RCS, ECS, and OCS, respectively). Each scheme is organized by a distinct set of principles and basic categories, which give rise to tensions and quandaries between them. Much of philosophy arises from the uneasy coexistence of the three schemes. It is interesting and illuminating to articulate what these three schemes are and how they relate to each other. We can imagine possible beings that live with only one of them, or possibly two, which would make cognitive life simpler; we, however, move around within and between the three schemes on a daily basis, suffering the intellectual cramps that arise. To put it crudely, the three schemes do not translate into each other; they adopt quite different viewpoints onto reality—perspectives, attitudes. Each can be studied separately and their interrelations mapped: they serve different cognitive ideals or governing concerns, generating different “worlds”, with a tendency to compete with each other for conceptual domination. It is as if our overall conceptual scheme has been designed with three different purposes in mind—a sort of unholy conceptual trinity.
In Individuals P.F. Strawson undertook to describe our conceptual scheme from a referential point of view: what general categories of things do we refer to and which of them are basic? He is clear that other sorts of inquiry into our conceptual scheme are possible, which may yield different orderings of conceptual priority. Following the referential method, Strawson finds that objects in space that can be identified and re-identified are the “basic particulars”, with events, processes, psychological states, and elementary particles emerging as non-basic particulars. Specifically, demonstrative reference to material particulars is basic in our RCS: we conceive of the world, reference-wise, in terms of such particulars and their relations. Thus we achieve an ordering of particulars, with some more basic than others. Maybe other conceptual schemes would refer to things differently, producing a different ordering; but our conceptual scheme favors persisting material objects in space—the kind of thing that can exist unperceived, has objective reality, and can be known about through perception. This implies that the basic particulars are open to skeptical doubt—their existence is not certain, despite their conceptual (referential) primacy. From the point of view of reference, material objects are primary, though they are not primary from the point of view of knowledge. Semantics and epistemology diverge.
Consider, then, the ECS: what is primary there? Not material objects but psychological states: the things we are most certain of are our own states of mind—thoughts, sensations, and so on. Here the ordering is based on the concept of inference (not referential dependency): what is basic is what constitutes the infallible foundation of knowledge and what is secondary is what can be inferred from that basis (validly or not). The epistemic ordering inverts the referential ordering: mental things first, material things second. A being that thought only from the epistemic point of view would regard anything inferred as secondary in its conceptual hierarchy, since there would be no competing pressure from the referential point of view to invert the ordering imposed by the epistemic point of view (similarly for a being who used only the referential scheme). But we operate both schemes, so we get a different ordering depending on which scheme we are considering. And isn’t that what generates skepticism? We refer (basically so) to things whose existence we cannot demonstrate. If we made no such reference, we would not recognize the existence of things that cannot be proven, so we would not be confronted with a skeptical problem; while if we made reference to material things, but didn’t think in terms of justification, we would also not face the skeptical problem.  Material objects are basic from a referential point of view but not from an epistemic point of view. Thus the two schemes sit uneasily side by side, committing us to the existence of things that we can see elude (demonstrable) knowledge.
Now consider the OCS: this scheme deals with questions of constitution, part and whole, what is made of what, ontological dependence. What are the basic constituents of reality? Some say invisible material particles, others say ideas in minds, and others plump for a neutral universal substance. What is basic in this scheme is what forms the basic stuff of reality—what there most fundamentally is. Notice that this question is not constrained by what is basic in the other two senses: the ontologically basic stuff might not be basic with respect to reference or with respect to knowledge. Someone who thought only in ontological terms would not even consider these alternative conceptual structures, so there would be no question of slotting them together: but we think in all three ways, so the question of harmony must arise for us. And what we find is that there is no alignment of categories between the three schemes: what is basic ontologically is not guaranteed to be basic referentially or epistemologically (atoms, say). What the world is fundamentally made of is not what we fundamentally refer to or fundamentally know—and that produces a tension because reference and certainty constitute ideals. It troubles us if we can’t know and refer to what is ontologically basic. Hence we find systems that attempt to integrate and reconcile the three schemes—the most obvious being idealism. Reality is constituted by ideas in minds, and ideas are the most certain things, and they are what language fundamentally refers to (“sense-datum language”). Russell had a view very like this, expressly geared to integrating reference and knowledge. Another type of view might be that reference is unreal and certainty is a chimera, but reality is thoroughly material (Quine)—we simply dispense with anything that fails to jibe with our ontology.  The trouble is that these monolithic systems do violence to our conceptual scheme, as it spontaneously exists—and which does generate real tensions. We have three different formats for representing reality, and they don’t agree on their conceptions of primacy. They privilege different things.
Persons have some claim to concentrate the unease most acutely: we refer to persons (they are basic particulars, according to Strawson), but we have trouble knowing that they exist (other minds) and trouble explaining what constitutes them (the mind-body problem and the problem of personal identity). They are primary in our scheme of reference, but they are not primary in our scheme of knowledge or our scheme of ontology: they are a matter of shaky inference and their nature is to be dependent (as well as obscure). Thus we make confident reference to things whose existence is uncertain and whose constitution is problematic. We conceptualize persons according to three different conceptual frameworks with nothing uniting these frameworks—no overarching conceptual structure. The question of whether selves exist at all results from these tensions: we proceed referentially as if they do, but when we look into the epistemology of the self it is elusive at best (Descartes, Hume); and no one can explain how selves are grounded in more basic facts about the world. We feel that we are firmly committed to selves by our RCS, but our ECS fails to ratify that conviction, and our OCS offers no help. If we conceived of persons in just one of these three ways, we wouldn’t feel so confused; it is the combination that gives rise to tension and puzzlement. Why are we so blithely referring to things whose existence we can’t demonstrate and whose nature we can’t explain? 
Sensations also illustrate the disharmony: they are primary for knowledge, but not for reference and ontology. Some go so far as to suggest that they cannot be objects of reference (qua private objects: Wittgenstein), and some flintily maintain that they cannot exist at all (eliminative materialists): they have a clear place in the ECS, but not in the RCS or the OCS. We are trying to think of them in three different ways simultaneously: as what is immediately and subjectively known, as what can be referred to in a public language, and as genuine constituents of objective reality. The attempt produces tension, discomfort, and intellectual cramps—the three schemes are not designed to dovetail neatly together. Each scheme has its peculiar point and use, and is harmonious within its own confines, but taken together we face a heterogeneous mishmash. It is as if the schemes come from different sources having little regard for anything outside of their own purview. Suppose that were so: suppose each scheme evolved at a different time in response to different adaptive needs—semantic, epistemic, metaphysical. We wanted to talk about things to each other, we wanted to describe our knowledge of things, and we wanted to think of things constitutively: so we separately evolved restricted conceptual schemes that would serve these diverse purposes. There was no attempt to regulate the contents of the three schemes in relation to each other; there was no grand design intended to harmonize them. They arose separately (genetically or culturally) and henceforth were required to coexist, easily or uneasily.  Philosophy attempts to organize a sort of truce between them, a way to harmonize them, but the task is not easy and always seems to produce procrustean results.
Thus we live with three conceptual schemes—or equivalently, one conceptual scheme with three distinct parts. They structure reality differently, as shown in the orderings they generate, and they are geared to different concerns—yet they are directed to the same world. Primordially, we are faced by experiences-of-things (the given): the three schemes each attempt to make sense of this basic fact. Thus we have reference to elements of reality, knowledge of reality, and what reality is intrinsically. Each has its own proprietary conceptual apparatus. What is basic within one scheme is not basic within the others, but we feel a pressure to reconcile the different orderings. It would all be so much easier if they happily coincided: if what was basic referentially was also basic epistemologically and ontologically. Then the things we knew best would be the most basic things in reality and also the things to which we referred most naturally—there would be perfect conceptual alignment (hence the appeal of idealism). As it is, however, there is mismatch and conflict—a kind of squabble between divergent viewpoints. The human mind is the scene of that squabble. We conceive of things from three contrasting viewpoints. Different philosophers choose to emphasize one viewpoint over the others (linguistic, epistemic, and ontological).
 A third type of position asserts that material objects are basic in all three senses: they are basic referentially, but they are also basic epistemologically and ontologically. This is because of naïve realism and scientific anti-realism: we know material objects directly as they are in themselves, not by inference from sensation, and unobservable entities like atoms are mere logical fictions or useful predictive devices. Middle-sized material objects are thus what we primarily refer to, what we primarily know, and what primarily exists. This position unifies the three parts of our conceptual scheme, thus eliminating the tensions: but it does so only at the cost of epistemological and ontological perversity.
 The logical order here would be: first referring to things, then asking how we know about them, then asking what constitutes them. These could occur in temporal succession. There need not be much continuity or consistency. Always remember that conceptual schemes are products of nature, mainly of evolution; they can be haphazard and makeshift.