"For indeed, all concepts have arisen from the senses – according to [1] circumstance, [2] analogy, [3] similarity, or [4] synthesis – with reasoning also contributing something." (DL 10.32)
Given that all our ideas are necessarily built only from impressions of the outside world, I do not understand how the idealist interpretation is tenable. We need impressions from external physical objects to form our thoughts. By analogy to direct impressions we are able to "mentally contemplate invisible realities." (DL 10.59)
Additionally, if an idea synthesized in our mind does not accurately correspond to an external object, then it is an empty opinion.
If you imagine a centaur, you have synthesized your impressions from reality into something that no longer corresponds to reality. In this case, the centaur exists in your mind as a real impression because it moves your mind with the impression of a centaur, but that synthesis does not correspond to reality (again, an empty opinion).
DL 10.49, 50 (Mensch Trans.) "We must also believe that it is when something from the external objects enters us that we see and think of them; for external objects could not stamp in us the nature of their own color and shape through the air that is between them and us, nor by means of the rays of light or any sorts of currents that travel from us to them, but rather by the entrance into our eyes or minds (as their size determines) of certain rapidly moving outlines that have the same color and shape as the external objects themselves; the same cause explains how they present the appearance of a single, continuous object and preserve their mutual interconnection at a distance from the substratum, their corresponding impact on our senses being due to the oscillation of the atoms in the solid object from which they come."
"And whatever image we derive by focusing the mind or the sense organs, whether on the object's shape or its concomitant properties, this shape is the shape of the solid object and is due either to the continuous compacting or to the residue of the image. Falsehood and error always reside in the added opinion [when a fact is awaiting confirmation or the absence of contradiction, which fact is subsequently not confirmed by virtue of an immovable opinion in ourselves that is linked to the imaginative impression, but distinct from it; it is this that gives rise to the falsehood]. For impressions like those received from a picture, or arising in dreams, or from any other form of apprehension by the mind, or by the other criteria, would not have resembled what we call the real and true things had it not been for certain actual things on which we had cast our eyes. Error would not have occurred unless we had experienced some other movement in ourselves that was linked to, but distinct from, the apprehension of the impression; and from this movement, if it is not confirmed or is contradicted, falsehood results; whereas if it is confirmed, or not contradicted, truth results. And to this view we must adhere, lest the criteria based on clear evidence be repudiated, or error, strengthened in the same way, throw all these things into confusion."