Schopenhauer imagined a dialogue between matter, which represents the viewpoint of materialism, and the subject of cognition, representing the viewpoint of idealism, to show that these views are not contradicting each other but are complementary parts of the whole of reality. This dialogue can be found in Schopenhauer’s handwritten legacy, volume 3, Adversaria no. 263.

What follows is my translation from the original German text.

Microcosm and Macrocosm:

a dialogue between matter and the subject of cognition.

The conflict of materialism and idealism, which is based on a flawed view, could be clarified by a dialogue between the matter and the subject, which drama could also be titled "microcosm and macrocosm", e.g. thus:

The subject. I am, and nothing is outside of me: because the world is my representation¹.

Matter. Impudent delusion! - I, I am, and apart from me is nothing. For the world is my temporary form: you are a mere result of a part of this form, and quite accidental.

The subject. What foolish conceit! Neither you nor the world would exist without me: they are conditioned by me. Whoever thought me away and believes that he can now still think the existence of those things, is subject to a gross deception: their existence apart from my representation is an outright contradiction, a wooden iron: they are: that is, they are represented by me: my representation is the place of their existence, and thus I am the first condition of it.

Matter. Fortunately, the presumption of your assertion will soon be refuted in a real way and not by mere words: still a few more moments, and you actually no longer exist, you have sunk into nothingness together with your grandiloquence, have floated by in the way of shadows and suffered the fate of every one of my transient forms. But I, I remain, unharmed and undiminished, from millennium to millennium, through infinite time and watch unshaken the game of the change of my forms.

The subject. This infinite time, which you boast of experiencing, is, like the infinite space that you fill, only present in my representation, indeed, is only a form of my representation, which I carry ready in me and in which you represent yourself, which receives you, through which you are there in the first place. But the destruction with which you threaten me does not affect me: otherwise, you would also be destroyed: rather, it only affects the individual who is my bearer for a short time and is represented by me, like everything else.

Matter. And if I grant you this and agree to consider, then your being, which is nevertheless inseparably linked to that of these perishable individuals, is to be considered as one existing for itself; thus, it nevertheless remains dependent on mine: because you are a subject only in so far as you have an object, and I am this object: I am its core and content, the abiding thing in it, which holds it together and without which it would float away as insubstantial as the dreams and fantasies of your individuals, which have even borrowed their apparent content from me after all.

The subject. You probably do not want to deny my existence because it appears in the individuals: for just as inseparably as I am chained to them, you are chained to your sister, the form, and have never appeared without her. No eye has seen you, like me, naked and isolated: for both of us are only abstractions and consequently entia rationis. It is basically a being that intuits² itself and is intuited by itself, whose being in itself is not constituted neither in intuition nor in being intuited, since both of these are divided between us.

Both. Thus, we are inseparably linked, as necessary parts of a whole that encompasses both of us and stands as a higher species above both of us: only a misunderstanding can make us hostile to each other and lead one to fight the other's existence, with which its own stands and falls.
This other, of a higher species, is the world as representation or appearance; with its removal only the Will³ remains, as purely metaphysical, as the thing in itself. But he who does not recognize the Will as such, may put an x here instead of it, which he may also name y, z, or however else he pleases. For, the present treatment stands for itself.

1 - Representation (Vorstellung) is a central term in Schopenhauer’s philosophy, adopted from Immanuel Kant. The German noun Vorstellung literally means "something put before". A representation is any object for a subject. Intuitive representations constitute our entire experience of the external world. Abstract representations ("representations of representations") are concepts, which are formulated by our faculty of reason, which constructs them from intuitive representations. →
2 - The term 'intuition' (Anschauung) is used by Schopenhauer to refer to any immediate and non-discursive experience or perception that could serve as the basis from which reason abstracts concepts. More specifically, intuition refers to our apprehension of empirical objects that stand in causal relations to other objects. All intuition is intellectual, but does not require thinking or discursive thought, that is, it is a function of the understanding, which automatically refers bodily sensations to spatio-temporal causes. →
2 - Schopenhauer viewed the will as the essence of the world, the ultimate substance, the agent in unconscious functions of organism, the common stuff of all being and the Kantian thing-in-itself. The world as representation is simply the mirror of the will. Everything is the will. The will expresses itself in phenomena from the forces of nature, such as gravity, through the deliberate conduct of humans. The will is one and outside the scope of space, time and causality. →

see also video Materialism and Idealism - A Dialogue