How did Penelope Maddy argue that sets can be perceived?

Penelope Maddy argued that sets can be perceived by rejecting the traditional Platonist claim that abstract objects exist entirely outside of space and time. Instead, she proposed that sets of physical objects are spatiotemporally located exactly where their members are located.12

Because these sets occupy space and time, Maddy maintained that humans can acquire knowledge of them through ordinary sense perception. For example, if you are looking at three eggs in a refrigerator, Maddy argued that you are not only perceiving the eggs themselves but also the set that contains them. In her view, you can know that the set has three members through the same process of observation used to perceive physical properties, such as the color of an egg. By situating sets within the physical world, Maddy sought to bridge the gap between human cognition and abstract objects, providing a way to explain how we gain mathematical knowledge without requiring nonphysical contact.21