We can know things only as they are related to us, under our forms of perception and understanding, insofar as they fall under our conceptual schemes, etc. So, we cannot know things as they are in themselves.
As Franklin comments, "...all there is to ... arguments ... [of this kind] is: our conceptual schemes are our conceptual schemes, so, we cannot get out of them (to know things as they are in themselves)." Or, Franklin continues, "[i]n Alan Olding's telling caricature, 'We have eyes, therefore we cannot see.'"
Franklin's essay, like Stove’s contest campaign, is a delightful skewering of skeptical pretensions. As long as you keep qualifying "things" to be "things in themselves," or "things as they exist unperceived in and for themselves," you can define your way right out of knowledge, and eventually construct "a seriously heavyweight argument, like: 'We can eat oysters only insofar as they are brought under the physiological and chemical conditions which are the presuppositions of the possibility of being eaten. Therefore, We cannot eat oysters as they are in themselves.'" (cf. Stove, 1991, The Plato Cult, pp. 151, 161)
This Worst Argument is at the heart of Kuhnian (and subsequent Bloorean) constructivist irrationalism in the philosophy of science. In response, Franklin argues that a mere causal story (i.e., a mere historical recounting of how various scientists reached certain conclusions), in no way negates the importance of rational truth in science and human thought in general.
So far it has been taken for granted that the invalidity of the 'Worst Argument' is obvious. That is because its invalidity is obvious: its conclusion, 'we cannot know things as they are in themselves' just does not follow from the premises about how we can know things only as they are related to us. Nevertheless, what is wrong with the argument can be seen more clearly through a parallel case. Take an electronic calculator. Why does the calculator show 4 when someone punches in 2+2? On the one hand, there is a causal story about the wiring inside, which explains why 4 is displayed. But the explanation cannot avoid mention of the fact that 2+2 is 4. On the contrary, the wiring is set up exactly to implement the laws of arithmetic, which are true in the abstract. The causal apparatus is designed specifically to be in tune with or track the world of abstract truths. If it succeeds, the causal and abstract stories co-operate, and the explanation of the outcome requires both. If it fails, the mistake is explained by some purely causal story. It is the same with brains that do science. For whatever reason, brains have the ability to gather reliable basic sensory information about the world. Information gathered by observation and experiment supports some scientific theories better than others, and a brain that draws the correct conclusions is performing well, while a brain that does not do well needs its mistakes explained.
The point that I take from this, vis-à-vis my recurring interests at FCA in rationality and neuroscience, is that reasons qua rational ends have an irreducible causal potency in the world, which is not to be reduced to, nor simply deduced from, bare causal effects. Because we can reason, and because reasons are indeterminate among their causal instantiations, we are free to act reasonably and purposefully. Or, as St. Thomas says in De Veritate (24, 2, resp.), "Totius libertatis radix est in ratione constituta [the root of liberty, whole and entire, is constituted in reason]."