Strange, because Kant is popularly associated with the "Thing in Itself", when he's associated with anything at all. On its surface the "Thing in Itself" is a very nihilistic concept. Kant believed that we can't perceive things in themselves but can only perceive processed appearances filtered through our mental machinery. Scientific equipment only helps partially, because human operators interpret the information through their own conceptual filters. It seems like we're pretty much stuck when trying to gain definitive knowledge of the world, right? I mean, we can't ever say that we've really gotten to what's truly behind things.
Well, not quite.
You see, one Kant's great insights, incorporated into all his philosophy, was that we ourselves are objects, and we possess consciousness, and because of this we can have insight into one Thing in Itself: our own. This can be done through self examination of psychological experience. Based on this fact, we can extrapolate that whatever's out there behind the things in themselves comes from the same root as human experience, human psychology, and human bodies. This is important because it means that, while not decisively knowable, some knowledge of the true nature of the world can be gotten by generalized analogy with interior human experience. We know that while the world may appear strange and different from us, much of it is natural, organic and living, and so probably shares similar characteristics with us to one degree or another. We can also surmise that life, or what we consider to be life, share's basic characteristics.
Kant's notion of personhood has been used by Tom Regan and others to justify animal rights based on similarities in psychology, but while these uses are perfectly valid, the concept can go much deeper. We have a personhood that entreats us to do unto others like ourselves based on our own experience of being human, but we also have primate hood, mammal hood, animal hood, as well as other hoods. As part of the components of nature, we potentially have insights into them that scientific observation might not be able to easily get to, the emotional experience of animals, for instance.
All of this matters quite a bit for philosophy, because philosophy is based on generalizations and the ability to make accurate generalizations that are connected to some sort of reality principle.
This possibility is what prompted later German idealist philosophers like Schelling and Schlegal to go towards a Naturphilosophie emphasizing nature as the ground of life and the start of philosophy, as well as towards Spinoza's concept of God as substance and all visible objects as variations on it, especially human beings. Significantly, according to Besier, these philosophers kept Spinoza's mode of thought close together with a subjective idealism presupposing radical self knowledge, because that was the surest way to get philosophical knowledge of the world. The position implies great freedom and autonomy for the individual within a somewhat determined schema, and prefigured much 19th century philosophy that integrated individual action within a structured social context that influences but does not determine it. In other words, in it's own way the Naturphilosophie was the forerunner of sociology, Marxism, and socialism in general, although Marx substituted economy for nature.