This text is of interest because it represents a point of contact between Locke’s empirical epistemology and the mundane concerns of utilitarian medicine. It is a point, so to speak, at which the theoretical meets the practical.
Locke points out that medicine, being an applied and concrete craft, was perhaps more susceptible to the natural, cultural, and subconscious influences which can distort reasoning. There might be a parallel here between Locke’s critique of medical thinkers and Francis Bacon’s identification of four sources of error in logic.
Because these earlier thinkers seemed unaware of these error-inducing influences, and took no measures against them, Locke finds most of what has been written about medicine to be, at the least, built on unsteady groundwork:
If, therefore the learned men of former ages employed a great part of their time and thoughts in searching out the hidden causes of distempers, were curious in imagining the secret workmanship of nature and the several imperceptible tools wherewith she wrought, and, putting all these fancies together, fashioned to themselves systems and hypotheses, ‘tis no more to be wondered at or censured that they accommodated themselves to the fashion of their times and countries, and so far complied with their most natural inclinations as to desire to have some basis to rest their thoughts upon, and some grounds to guide them in the practice of their art. Their being busy and subtile in disputing upon allowed principles was but to be employed in the way of fame and reputation and the learning valued in that age; and that their practice extended no farther than the sacred principles they believed in would permit, is no more to be admired than that we find no fair and lasting fabrics left to us by our ancestors upon narrow and unsound foundations.
Locke hastens to add that he respects these earlier writers, despite the flaw in their writings which he is identifying, because they catalogued a significant body of observations, and developed laws based on correlation.
Those writings, however, are saturated with unmerited conjectures which, left uncorrected, will lead readers astray:
I would not be thought here to censure the learned authors of former times, or disown the advantages they have left to posterity. To them we owe a great number of excellent observations and several ingenious discourses, and there is not any one rule of practice founded upon unbiased observation which I do not receive and submit to with veneration and acknowledgment; yet I think I may confidently affirm that the hypothesis which tied the long and elaborate discourses of the ancients, and suffered not their enquiries to extend themselves any farther than how the phenomena of diseases might be explained by those doctrines and the rules of practice accommodated to the received principles, has at last but confined and narrowed men's thoughts, amused their understanding with fine but useless speculations, and diverted their enquiries from the true and advantageous knowledge of things.
Locke seems, then, to be engaged in a Baconian task: freeing an observational empirical science, or the applied form of it, from systematic methodological shortcomings which nudge it toward error, or at least toward unfounded hypotheses.
The reader might further wonder if Locke were directly inspired by Bacon’s texts to this task, or whether he were indirectly inspired as Bacon’s influence came down through other thinkers like Robert Boyle, or whether Locke happened upon the same concerns independently by coincidence. Peter Anstey writes:
There is sufficient evidence to claim that Locke owed a significant debt to Bacon's conception of how natural philosophy should be done.
Concerning the connection, if any, between Locke and Bacon, Anstey notes:
So it would appear that Locke’s comments on method in natural philosophy, whatever their peculiarities, stand in a tradition stretching back to Bacon. Yet surprisingly, John Locke and Francis Bacon are not normally associated with each other. There are few references to Bacon in Locke's writings and the weight of scholarship seems to lean to the view that Bacon had little, if any, influence on Locke.
Anstey goes on to cite an article by John R. Milton about the influence of Bacon on Locke:
It should be pointed out that Milton also claims that the quantity of Bacon’s books owned by Locke is “strong prima facie evidence that Locke was interested in the thought” of Bacon and that parallels in the intellectual content and stylistic expression of Bacon and Locke suggest Bacon as a source of influence on Locke.
That there are points of comparison between Locke and Bacon is clear; there are doubtless points of contrast, as well. It seems that at least some of Bacon’s influence on Locke was direct; we know that Locke was aware of Bacon and had read some of Bacon’s texts.
The flaws which Locke notes can be understood to correlate with Bacon's four sources of experimental error.
There should therefore be no surprise that Locke’s writings about medicine have a Baconian flavor.
