The Puzzle of Empiricism
Even the most ardent opponent of empiricist epistemology will concede that it is not wholly wrong. It may be that much knowledge is not based on experience, and it is no doubt true that knowledge requires more than mere experience, but surely it is undeniable that some knowledge comes from the senses. Not all knowledge is a priori; some things we can only know by experience. You can only know by means of the senses that the sky is blue or that it’s raining or that all birds have feathers; reason alone cannot assure you of these truths. For the ascertaining of some facts you need to employ your senses; nothing else will do. There is room for debate about what exactly the empiricist doctrine maintains—what is meant by “experience”, and what is it about the senses that is so essential? Do we mean the actual human senses or can we include other non-human senses, real or imaginary? Is it experience as a subjective state of consciousness that is necessary? Thus we might allow for Martian senses different from ours, and we might relax the requirement of conscious experience so as to take in people with blindsight (maybe they can have empirical knowledge and yet never experience conscious sensations of external objects).  Perhaps the doctrine should be formulated not in terms of experience as a conscious state but in terms of causal interaction with external objects: there is a type of knowledge which is such that it can only be acquired by causal interaction with the objects of that knowledge. But putting these refinements aside, it seems indisputable that some knowledge is possible only by using the senses; pure ratiocination is never enough. Humans and animals have senses for a reason: the senses are what enable them to know about the world around them—nothing else will do the job.
It may be wondered how strong this claim is: is it a necessary truth that certain facts can only be known by sense experience (to revert to the traditional formulation)? What if Plato is right about some possible beings that know everything by means of recollection from a previous life?  They never experience anything themselves (in this life) but know about the natural world by remembering what their previous life revealed (presumably by experience). Or perhaps there could be beings whose genes encode a lot more information than ours—containing an encyclopedia of knowledge concerning what for us is known by post-natal experience. These beings are simply born knowing that the sky is blue and that all birds have feathers. This doesn’t seem logically impossible. So maybe empiricism is true of us and beings like us but not true of all possible knowing beings. But even these counterfactual possibilities don’t undermine the essential thrust of the empiricist doctrine, because someone had to interact with reality in order to lay down the knowledge in question, either in intergenerational memory or in the genes. There had to be some sort of imprinting of reality on minds or brains, not just a priori cogitation: for surely it is impossible to figure out by reason alone that the sky is blue or birds feathered. You need to have a look. The facts have to impress themselves on your sensorium, shaping your view of things: at a minimum some sort of causal connection is required. The paradigm is vision: the visual apparatus has to be activated by external facts and experiences thereby generated—these being the basis for justified belief. Empiricism is the doctrine that nothing else can act as the source of certain kinds of knowledge. Testimony can supply it, to be sure, but only because at some point someone cast a sideways glance at the world. In short, experience is the root of (and route to) knowledge of the external world. This is a necessary (and indeed a priori) truth.
Although I have strong rationalist leanings, this at least seem correct to me: there is a large grain of truth in empiricism. Historically, empiricism was set against rationalism and certain forms of intellectual authoritarianism; the latter was perhaps the more powerful motivator. Experience is a better source of knowledge than the Bible, the Church, or Aristotle’s writings. Here the empiricists had an invincible argument at their disposal, though I don’t recall them ever using it, namely that we can only know what is in the Bible, or what priests say, or what Aristotle and his interpreters say, by sense experience. We have to look and see, or listen and hear, in order to find out what these supposed authorities maintain. So even scriptural knowledge is based on personal sense experience (as is true of all testimony knowledge). And surely it is uncontroversial that rationalism is hopelessly implausible if offered as a general theory of human (and animal and alien) knowledge—which is why no rationalist has ever tried to claim any such thing. The question was just whether there is any knowledge apart from sense-based knowledge—for there is surely a great deal of knowledge that is derived from the senses, necessarily so. It is not an unwarranted dogma of empiricism that human knowledge includes sense-based knowledge! On that point we can all agree (accepting the emendations that might be required when we start to explore logical space).
But that is not the end of the story, because a question remains outstanding: why is this so? What is the explanation of the fact that some knowledge can only be acquired by empirical means? There exists a partition of facts such that one side consists of facts known by pure reason and the only side is known (and can only be known) by means of the senses: but what distinguishes these facts? This is what I am calling the puzzle of empiricism—the puzzle of why it is that certain facts call for sense-based knowledge, as a matter of a priori necessity.  Why is it that the fact that the sky is blue is necessarily only knowable by means of perceptual experience? What is it about that fact that makes this so? How does the ontology dictate the epistemology? What makes these facts special in this way? Granted, it is quite true that they can only be known in the empirical way, but what makes this the case—why this restriction on the mode of knowing? It is sometimes maintained, with some plausibility, that mathematical propositions can be known both a priori and a posteriori, but that propositions about the external world can only be known empirically; the former seems like the natural states of affairs, since ontology cannot dictate epistemology, but the latter also seems indisputable. This is puzzling. The empiricist owes us an answer to the question of why fact and knowledge must line up in this way. To put it bluntly, why are some facts such that they can be known only by being experienced? Not all facts have this property (a priori facts), so why do some? Isn’t it strange that the world should have written into it that it can only be known in a certain way? How can it be that God had no degrees of freedom with respect to epistemology when he decided on the ontology of material objects with properties? Experience is not always required for knowledge, so why in this instance? It isn’t as if the facts in question are experience!
Here is the kind of answer we might hope for: empirically known facts have a different structure from facts known a priori. In the former case the predicate of the fact is not contained in the subject of the fact, while in the latter case it is. The facts differ ontologically, and in a deep and principled way. We can know truths of the a priorikind by recognizing the containment relation between subject and predicate (what Kant called “explicative knowledge”), whereas we have to go beyond the subject to ascertain the predicate in the empirical case (Kant’s “ampliative knowledge”). Now this may or may not be an adequate account of the matter, but I think it is clear that it doesn’t answer our question, since it is obscure why lack of containment should necessarily require knowledge by sense experience. After all, we have the category of the synthetic a priori, so lack of containment is consistent with the absence of an experiential basis. Nothing about an “ampliative” fact entails that it should be knowable only by experience. So there is nothing here about the fact as such that correlates with being knowable only empirically. Nor would it be right to say that the fact is precisely a material fact, unlike the facts of mathematics: for (a) the same points apply to facts of psychology, and (b) there are material-object facts that are known a priori, e.g. the fact that all material bodies are extended, or the fact that Hesperus is self-identical. So we are still bereft of any distinguishing feature that explains the epistemological necessity we are puzzling over. We still don’t know what makes a fact knowable only by means of sense experience. Thus empiricism remains curiously puzzling, though apparently perfectly true.
An alternative explanation invokes the notion of intelligibility. Some facts are inherently intelligible, it may be said, but some are not. Facts known a priori fall into the former category while facts known a posteriori fall into the latter category. If every fact in the world were internally intelligible, then all knowledge could be a priori; but that is not actually the case. Accordingly, not all facts can be known a priori, and empirical knowledge steps in to take up the slack: it saves the day when the rational faculty struggles with an absence of intelligibility. Thus the senses are our only means of grasping a non-intelligible world—a world of contingency, accident, and happenstance. Empiricism therefore follows from the lack of universal rational intelligibility. The trouble with this explanation is, again, that it is not clear why the alleged contingency of the world should require a sensory faculty to take in its accidental nature. Why couldn’t there be another way to know these non-intelligible facts? What is it about brute facticity that requires the operation of the senses—those specific organs for gaining knowledge? It doesn’t require any specific sense, so why any sense? Does God need senses to know about the world just because it has an accidental character? Also, there can be no denying the centrality of vision in the empiricist outlook; the other senses play at best a subsidiary role. It is difficult to see how we could have acquired our extensive empirical knowledge of reality by relying only on our non-visual senses. So is it that the non-intelligible nature of the world demands a visual sense in order to be known (or at least one analogous to vision)? Thus we derive “ocular empiricism”. But the contingency of the connection here undermines any idea of logical necessitation from fact to faculty. Empirical knowledge is certainly not identical to the experiences that prompt it, but builds upon such experience; the experience triggers the knowledge. But then why couldn’t something else trigger it—as testimony can? As long as it is triggered somehow the knowledge results, so the sense experience seems logically redundant; it isn’t a conceptually necessary condition of having the knowledge in question. Again, the truth of empiricism seems strangely ungrounded—not at all transparent. It is (dare I say it?) mysterious.
And there is this further point: sense-based knowledge is far from ideal from an epistemological point of view. So it isn’t some unimprovable type of knowledge that steps royally in to deliver the goods. On the contrary, it is subjective, extremely limited, in need of supplementation, susceptible to skeptical assault, and downright primitive (no doubt deriving from our animal ancestors: it is the way fish know things). Not for nothing are the senses denigrated in comparison to Reason. So empiricism is saying that our knowledge of certain facts is necessarily mediated by a pretty dismal epistemic faculty—poorly designed at best, riddled with error at worst. Why should this be what the objective world demands in the way of methods of knowing about it? Isn’t it rather insulting to the objective facts to say that they are condemned to be known by such a puny and flawed collection of faculties? What if we only had smell and taste to go on—wouldn’t empiricism seem troublingly hobbled? Doesn’t the world deserve something grander, more hi-tech, and more penetrating? At least reason seems to live up to its object, giving real insight into what it makes known, but the senses just graze the surface—couldn’t there be a better way to know the facts in question? Empiricism elevates to the epistemic pinnacle what is in truth a lousy way of learning about the world. Maybe we are stuck with it, just miserably out of luck, but it hardly qualifies as the epistemic be-all and end-all. It is beginning to appear almost paradoxical—not merely puzzling and mysterious—that the senses are the only conceivable route to knowing about the natural world. It is just inexplicably bad epistemic engineering. What is puzzling is that it still seems quite true that we can only know external reality by sense experience. Those facts will grant epistemic access only to the senses as methods of gaining insight into them, for reasons hard to fathom. The undeniable (and large) grain of truth in empiricism thus hides a mountain of mystery. 
 We can’t help noticing that empiricism itself is contrary to its own tenets: for how can it be a matter of empirical knowledge that empiricism is an a priori necessity? We might then view total empiricism as self-refuting—which no doubt it is (though this is always a rather cheap objection to a philosophical theory). As Hume would say, where is the impression that corresponds to our knowledge that empiricism is an a priori necessity? In fact, of course, the theory is offered as a piece of (speculative) ratiocinative knowledge in the grand rationalist style.
 Is it that the self-evidence of empiricism, construed in the restricted way suggested, has blinded us to asking why such a surprising thing should be true? It seems so obvious that we can only know that the sky is blue by experience that we forget to enquire into the rationale for this fact. Certainly, the classical empiricists seem quite untroubled by the question, not even raising it (to my knowledge).