"The thing of which the act of perception is the perception is experienced as something not mental"

In this quote, Samuel Alexander discuss a crucial problem in the viewpoint of perception, particularly the nature of perceptual experience and the difference in between the mental and the physical. His words recommend a philosophical position referred to as "naive realism" or "direct realism", which posits that perception allows us direct access to things worldwide, not merely psychological representations or constructs of those items.

When Alexander refers to "the thing of which the act of understanding is the perception", he is speaking about the item or event in the external world that is being viewed. Simply put, if you are looking at a tree, the "thing" would be the tree itself. Alexander stresses that this "thing" is experienced as something "not mental". This suggests that the act of understanding provides an experience of the tree as it exists on the planet, independent of our psychological activity or interpretations.

This position is contrasted with other theories of understanding, such as indirect realism and idealism, which argue that we just ever perceive psychological images or concepts of objects, rather than the objects themselves. Indirect realism posits that understandings are mediated by psychological representations, while idealism suggests that whatever we view is constructed by the mind.

Alexander's assertion challenges these perspectives by preserving that when we view a things, we encounter it straight, in its physical form. This view emphasizes the independence of the external world from our understandings, recommending that items keep their homes despite whether they are being perceived.

However, Alexander's declaration also implicitly acknowledges a duality in perception: there is the psychological act of viewing and the non-mental "thing" being perceived. This duality raises important philosophical concerns about the relationship in between perception and reality and how our minds interact with the world. Alexander motivates reflection on the clarity and immediacy with which the world presents itself to us, advising a reassessment of the limits in between the mind and the external world.