It's less about what's vaguely different between those two, and more about what's commensurable or incommensurable for them. What epistemic graph theory suggests, for example (in its "semantics," which is, however, often merged with its syntax), is that reductionism could be interpreted as a specific kind of epistemic graph,E which is structurally very similar, perhaps on some level even isomorphic to, foundationalism, which would answer your question in the affirmative on its own other level.
Alternatively, then, we might yet rank foundationalism as deeper than reductionism, in part on account of the direction-of-action: foundationalism proceeds from seeds to crowns of flowers and leaves, reductionism is a little ant crawling down the tree to see the roots. So there is a context in which it is reasonable to judge these doctrines sufficiently distinct to merit claiming the possibility of all permutations of absence and presence, here: either one is true and the other (whichever) isn't, neither is true (not always true, that is...), or both are true; so 4 options total.
Historically-speaking, for what it's worth, we should add in some of sec. 3.3 of the SEP article on the Vienna Circle:
How then can Vienna Circle philosophy be absolved of foundationalism? As noted, it is the Aufbau (and echoes of it in the manifesto) that invites the charge of phenomenalist reductionism. To begin with, one must distinguish between the strategy of reductionism and the ambition of foundationalism. Concerning the Aufbau it has been argued that its strategy of reconstructing empirical knowledge from the position of methodological solipsism (phenomenalism without its ontological commitments and some of its epistemological ambitions) is owed not to foundationalist aims but to the ease by which this position seemed to allow the demonstration of the interlocking and structural nature of our system of empirical concepts, a system that exhibited unity and afforded objectivity, which was Carnap’s main concern. (See Friedman’s path-breaking 1987 and 1992, and Richardson 1990, 1998, Ryckman 1991, Pincock 2002, 2005. For the wide variety of influences on the Aufbau more generally, see Damböck 2016.) However, it is hard to deny categorically that Carnap ever harbored foundationalist ambitions. Not only did Carnap locate his Aufbau very close to foundationalism in retrospect (1963a), but a passage in his (1930) led Uebel (2007, Ch.6) to claim that around 1929/30 Carnap was motivated by foundationalist principles and reinterpreted his own Aufbau along these lines (around the same time that Wittgenstein entertained a psychologistic reinterpretation of his own Tractatus that was reported back to the Circle by Waismann). To correct this foundationalist aberration was the task of the Circle’s subsequent protocol-sentence debate about the content, form and status of the evidence statements of science.
What this gets at is that reduction-to-sense-data, for example, has a foundationalist gist to it: observation reports of sense data purportedly being infallible and fundamental, they then go on to form (if via inductive reasoning) axioms for deductions therefrom, or more poetically of the "logical construction of objects from sense data" (in Russell's sense(!)). A reduction of macroscopic to microscopic physics, on the other hand, might not seem especially concerned with the direction of the evidence as such, or proceeds just as willingly from quantum information to biological spacetime, and then back and forth further and in more directions, etc. etc. forever and ever, amen. Now, though, we find a distinction between reductionism and foundationalism whereby the former is, in part, a subtype of the latter: sense-data/logical-construction theory, which is reductionist on account of being foundationalist to boot.
EOne might say: foundationalism is to reductionism what coherentism is to holism, to some extent, e.g. possibly isomorphic (see the remainder of the above).
Further reading:
- Tho[20], "Mechanical Philosophy: Reductionism and Foundationalism."
- Berker[13], "Coherentism via Graphs."
- Mullins[19], "Infinite Cycles and the Graphical Approach to
Epistemic Justification."
