To Hume in line with Kant, this supposedly 'unknown something' is a fiction of the mind;
https://iep.utm.edu/hume-ima/#SH5d
What is the philosophical realists who cling to a noumenon respond to the above?5d. The Philosophical Fiction of an Underlying Substance
Hume thinks that an ordinary sensible object, like a peach or melon, is just an aggregate of sensible qualities: for example, a ripe peach is an aggregate of a yellow-orange color, a fuzzy texture, solidity, and a sweet smell and taste (T 1.4.3.2, 1.4.3.5; SBN 219, 221).
However, he thinks that we are prone to suppose otherwise.
Instead of taking a peach to be an aggregate of many sensible qualities, we take it to be one thing.
This leads to a kind of philosophical puzzlement: how can many things (the many aggregated qualities) also be one thing—isn’t this an “evident contradiction” (T 1.4.3.2; SBN 219)?
According to Hume, many philosophers have responded to this puzzle by supposing that a peach is not the same thing as its sensible qualities, but is instead “an unknown something”—a substance or substratum that underlies its sensible qualities, and in which those qualities exist.
The presence of this “unknown something,” underlying the sensible qualities, is what gives the peach “a title to be call’d one thing” (T 1.4.3.5; SBN 221).
Hume thinks that this underlying substance or “unknown something” is a fiction, characteristic of ancient philosophy (T 1.4.3.1, 1.4.3.5; SBN 219, 221).
It is “feign[ed],” or postulated, by the exclusive imagination.
Hume also calls this fiction an “unintelligible chimera” (T 1.4.3.7; SBN 222).
Elsewhere, he explains the sense in which it [unknown something] is unintelligible.
All of our ideas are copied from our impressions, or are made up of ideas that are so copied.
But an underlying substance is supposed to be an entirely different kind of thing from an impression.
So, we cannot form an idea of an underlying substance (T 1.4.5.3; SBN 232–3).
Does this mean that we cannot think about underlying substances at all?
When Hume introduces the concept of an idea, he equates having ideas with thinking.
This suggests that the answer is yes—the fact that we cannot form an idea of an underlying substance does mean that we cannot think about such substances at all. [Kant – can think of the noumena]
However, other things that Hume says cast doubt on this interpretation.
He seems to posit several different fictions that cannot be made up of ideas copied from impressions.
For example, the “unintelligible” fiction of an underlying substance differs from the “incomprehensible” fiction of a perfect standard of equality (T 1.2.4.24; SBN 47–49).
But how can entertaining one of these fictions differ from entertaining the other if, in each case, we have no thought at all about the thing that we are feigning, or fictitiously representing?
Some commentators solve this puzzle by pointing to passages where Hume seems to distinguish two kinds of imaginative thought: conceiving and supposing (T 1.2.6.8–9, 1.4.2.56; SBN 67–68, 218).
Hume seems to equate conceiving with forming ideas (T 1.2.2.8; SBN 32).
This leaves open the possibility that supposing is a kind of imaginative thought that does not involve forming ideas.
If this is Hume’s view, then he can allow that we can think about underlying substances and perfect standards of equality by making suppositions about them, even though we cannot conceive them or form ideas that represent them.
For an interpretation of this kind, see Wilbanks (1968).
For a helpful discussion of the problem posed by “unintelligible” fictions, and a creative solution, see Loeb (2002: chapter 5, esp. 162–72).
Other Views?