Plato strove to explain this very concept, among others, in his Theory of Forms.
Without getting too detailed, a Form is an abstract quality or property that transcends space and time. For example, if you take a basketball and think only of its roundness, you are observing the Form of Roundness. The Form is separate from the basketball, and all round objects participate or copy the Form of Roundness. You can likewise say this about the ball's color or size.
The world in which we live is basically a shadow of the world of Forms. A great way to explain this is to visualize a cave with a fire inside it. As people and objects move around the fire, their shadows are cast on the cave wall. Our world is the cave wall, with material objects being crude shadows of the Forms, which are what is truly real.
Because we all participate in these Forms, we are aware of all objects, whether or not we think of them consciously. This is why ancient civilizations were able to independently develop techniques for building homes and structures with fairly similar designs. If you look at a window built by multiple ancient civilizations that were isolated from each other, they all look pretty much the same. Another example is how regardless of style or design, all of us can recognize a chair or a table. Essentially, no idea is original or really ours. We simply participate in a higher, more perfect world of Forms.