Idealists don’t believe only our minds exist (that is known as solipsism). The world exists, they say, but as a mental construct, and only when we perceive it. To use a different analogy, think of your refrigerator light. Whenever you open the door, it’s on. You might conclude that it is always on, but that would be a mistake. You don’t know what happens when the door closes. Likewise, we don’t know what exists beyond our mind’s capacities of perception.