Final answer:
In the face/vase illusion, the key to seeing both images is altering the figure-ground relationship, which involves distinguishing between the positive shapes (the figure) and the negative shapes (the ground).
Step-by-step explanation:
In the classic face/vase drawing, one can see both the face and the vase by being able to reverse the figure-ground relationship. This relationship is about differentiating between the positive shapes, which are the main subject ('the figure'), and the negative shapes, which are the background ('the ground'). To perceive one image or the other, one must shift focus from viewing the faces (figure) as the positive shapes and the vase (ground) as the negative space, to viewing the vase as the positive shape and the spaces around as the negative.
Visual perception's duality in such drawings offers an excellent example of the complex vantage point used by artists throughout history. Whether depicting scenes from myths and daily life on vases or capturing the intricacies of narrative elements, artists have always played with positive/negative shapes and figure/ground relationships to create depth and meaning.