No reason justifies disbelief.

[The Grand Nudger]

Okay.  All fair points. But, perhaps "knowledge" is a misnomer when talking about these in utero instincts or reflexes. You're right; I don't "know" how to smile because I spent several weeks walking around watching other people on the street smile, and then practiced in the mirror until I got it right, but that doesn't turn these neurological reflexes into something more than the purely physical effects of a physically developing neurological system, just like it wouldn't be quite right to say that a fetus responding to light in the womb "has knowledge" of how to kick its legs. It's a learned, physical ability.

Learned......by what means?  Not anything empirical as envisioned.  This amounts to non empirical knowledge unless you have some specific definition of knowledge that argues compellingly to the contrary or that that this x is fundamentally different from knowledge.  

Even still, the capacity for these reflexes is still intertwined with sensory experience, severely limited though they may be in early life. The fetus is responding to sensory input (light) in the example I just mentioned. And, it is suspected that many fetuses dream of the sounds, tastes, and sensations they experience in utero at quite an early gestational age. There is no tabula rasa. Humans are physical beings that begin having some base level of experience as soon as the neurological system is mature enough to start processing its external environment. The learned physical abilities of premature neurological development can hardly be conflated with intuitive, metaphysical knowledge, and I find it hard to believe that that is what most intuitivists mean when they're talking about intuitive knowledge versus empiricism. They still necessarily depend on a physical brain capable of experiencing, on some level, whatever exists outside of it. I suppose I could possibly get on board with this idea you mentioned, nativism, as at least it doesn't seem to be proposing some kind of woo-substance outside of the natural world. Either way, I clearly suck at this. What is your best argument for empiricism as the foundation of knowledge?

The things we're born with are exactly the kind of things that innate or nativist knowledge theorists are thinking of.  If we possess those then empiricism as conceived was wrong.  

My best argument?  Not so different than your own (or even at all), my best argument is to accept that nativist and innate knowledge was and is an actual thing, even if we didn't have the specifics entirely accurate.  Empirical experience and knowledge are translatable and communicable.   Those things we all do, these common human bits of knowledge expressed as behavioral norms -are- the product of sensory experience.  Just not our own.  They were shaped and powerfully informed by a long process of accurate and inaccurate empirical observations that resulted in the very structure of our brains as passed to us in a genetic inheritance.  

I know to, and how to smile and run because there are very few creatures left alive that didn't.  It is innate, and I didn't have to form this from any personal experience, it's the baseline of successful cognitive architecture.  Just like my assumed absolute knowledge of axiomatic principles.  I do think that some of the things we commonly put down to intuition are much the same. Heuristsics for reproductive success that just so happen to coincide with true postulates.

I suspect that there used to be many more that didn't...but those motherfuckers are all dead. That's what happens when your intuitive, innate, or derived knowledge is meaningfully in error. You end up as dinner rather than the diner.