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Treatise I & EHU
It would be a mega-task to work out the full relation of Treatise I to EHU - and the outcome certainly not beyond challenge. The following extract from an article by Phillip Cummins throws some light, however. Four main point are involved :
... In closing may I suggest that [1 - GLT] Hume disowned the Treatise primarily
because it provided handles for attacks on him and because he came
to abhor its brash and dogmatic style. This is not, in itself, an explanation of the differences between the Treatise and Enquiry. [2 - GLT] What seems
likely is that the latter was designed to carry forward more fully and
more successfully the program of the Abstract of a Treatise of Human
Nature. Hume by no means repudiated the Treatise in the Abstract,
but he did acknowledge that its length and abstractness made it
difficult to comprehend. [4 - GLT]To remedy this-that is, to make his
philosophical system more accessible-he decided to limit his discussion
of the Treatise to one central and important topic, causal inference,
and to provide only hints regarding "particular passages which seemed
to me curious and remarkable. Now the Enquiry develops along
similar lines. Hume concentrated on causal inference. After providing the needed introductory material (Sections One through Three),
he proceeds to offer his analysis of such inferences (Sections Four
through Seven) and then apply it to a number of related topics
(Sections Eight through Eleven). Section Twelve provides hints
regarding curious and remarkable doctrines. There are a number
of links connecting the Abstract and Enquiry - for example, the
introduction of Adam with his faculties perfected and the introduction
of the billiard balls, so there is some basis for the suggestion that the
former was the model for the latter. This hypothesis, at least, has
the advantage of not requiring the claim that Hume repudiated
doctrines of the Treatise which happen to be omitted from the
Enquiry.** ( Phillip D. Cummins, 'Hume's Disavowal of the Treatise', The Philosophical Review, Vol. 82, No. 3 (Jul., 1973), pp. 371-379: 378-9.)
Let's descend to detail just for illustrative purposes to EHU, Section 12:
Hume, of course, did not jam all of Part Four of Book One of the
Treatise into Section Twelve. They do, however, have a common
theme: the uses and abuses of skepticism. Furthermore, the uses to
which Hume put at least some of the doctrines developed in detail
in the Treatise conflict with ... [the] claim [sometimes made] that by the time of the
composition of the Enquiry Hume had repudiated metaphysics. In
Part Two of Section Twelve, Hume argued that mathematicians
assist skeptical attempts to turn reason against herself with their
demonstrations that finite magnitudes and durations consist of an
infinite number of parts. He added, in a footnote, that such an
absurdity can be avoided if one holds, "that there is no such things
as abstract or general ideas," and insisted, in another footnote, that there are "parts of extension, which cannot be divided or lessened,
either by the eye or imagination." Both claims were defended at
length in Book One, Part Two, of the Treatise. In Part One of Section
Twelve, Hume summarized his attack on the primary and secondary
qualities distinction in Section Four, Part Four, Book One, of the
Treatise. That attack was based upon the doctrine of colored and
tangible points expounded in opposition to the doctrine of infinite
divisibility. Hume also retained in Section Twelve the devastating
critique of both natural and philosophical beliefs in external objects
which was first developed in Section Two of Part Four of Book One.
He merely omitted detailed causal explanations of those beliefs.
The metaphysical doctrines of the Treatise were not repudiated;
rather, the detailed psychological arguments concerning the causes
of beliefs were dropped. Strange behavior for one fleeing the corruption
of metaphysics. (Cummins: 377-8.)
An abstract of a book lately published: entitled a treatise of human nature' & EHU
A light illustration of the connection : There are a number
of links connecting the Abstract and Enquiry - for example, the
introduction of Adam with his faculties perfected and the introduction
of the billiard balls, so there is some basis for the suggestion that the
former was the model for the latter.
But more in detail :
The title page of the Abstract of the Treatise tells us that in it "The
Chief Argument of that Book is farther illustrated and explained".
The contents of the Abstract show the "chief argument" to be the
theory of causal inference presented in the central sections of Part III
of Book I of the Treatise. Thus we may know what Hume himself
regarded as being of basic significance in his new philosophy. (Ralph W. Church: Review of An Abstract of a Treatise of Human Nature by David Hume, J. M.
Keynes and P. Sraffa, The Philosophical Review, Vol. 48, No. 6 (Nov., 1939), pp. 643-644: 644.)
It is a small step from recognising the Abstract as pointing out the matters of principal importance in T Bk I to seeing EHU as setting out just those matters more concisely, more precisely, and altogether more accessibly than the argument and exposition of the Treatise had managed to do.