Can someone explain Nietzsche's and Stowe's argument against Idealism?

Idealism holds that existence is a product of perception. To be is to be perceived.

This is a confusion of equating what is experienced with the experience of something. Changing a particular experience of something, doesn't imply a change in how that something is to be experienced.

The first argument, or at least dent, against idealism came with Kant, who contended that although we can't perceive anything other than our perceptions, that doesn't deny the existence of objects that are not perceived, but which we can infer are the cause of our perceptions, and therefore our knowledge. To argue for this, Kant devises what he calls a priori forms, such as space and time, which according to him, exist before the perception and are needed, together with experience, to apprehend it.

This pissed off Nietzsche, who called Kant on begging the question. What, according to Kant is an obvious truth, that what we perceive are perceptions, Nietzsche argues that "a faculty" of perception is assumed and used to argue for the occurrence of perceptions. He is saying that by saying that perceptions are perceived he is just restating the question of experience. After all, what is meant by "perceiving" is not explained away by saying that it's what you do to acquire perceptions.

Stove restated this more firmly by showing the precise slip in logic. He sates that the mistake is made by proceeding form the statement that nothing can be thought of which isn't thought of, to assuming that nothing can exist without being thought of. This is the same argument Nietzsche makes.

Russell also chimes in, with the same argument, stating that being "in" the mind, as ideas are, doesn't follow that they are "of" the mind, as idealists claim.