The primary qualities of an object do not depend on the existence or opinions of an observer. For example, the Grand Canyon existed long before humanity and, though it has changed over time, as all things do, has generally maintained its shape. Each of its sections has a specific depth and width as well as shape, which can be measured and reproduced. Secondary qualities, on the other hand, are the thoughts that the objects generate in people, which vary between different observers. In the Grand Canyon example, a layperson observer may see it as majestic or terrifying, while a geologist may be fascinated by the research potential that it provides. In some cases, secondary qualities can be related to primary ones, such as when a person tries to estimate an object’s size, while in others, such as that above, there is no “correct” perspective to check views against.
Under the theory of representative realism, there is no way for an individual to confirm that the world beyond their mind actually exists. All people can do is try to rationalize the ideas that appear in their thoughts, as those are their only potential way of interacting with the real world. However, as the question notes, it is known that in some circumstances, people can perceive objects that do not exist in others’ perceptions. It is reasonable to assume that those objects do not exist because otherwise, one would have to accept the existence of a multitude of almost non-perceivable objects and entities in the world. This assumption further raises the question of whether any objects experienced by people are real. The only way of potential confirmation is the creation of a majority consensus on reality, but philosophically, it is not sufficient to reach a conclusive answer.