Final answer:
Kant argues that consciousness of one's own existence necessitates the perception of external things because the categories through which we understand the world apply only to the empirical world experienced in space and time.
Step-by-step explanation:
According to Immanuel Kant in The Refutation of Idealism, the existence of external things is necessary for the determination of one's existence in time because external things are essential for the consciousness of one's own existence. This positions falls into Kant's broader philosophical perspective that combines both empirical (a posteriori) and rationalist (a priori) approaches to knowledge.
Our understanding of objects, including their spatio temporal properties, is bound by the innate categories of thought, which provide the necessary rules for thinking. As these categories apply only to the empirical world and since our consciousness apprehends objects in space and time, the determination of one's existence in time is intimately linked to the perception of external objects. Kant emphasizes that while we are keenly aware of our existence, to fully comprehend our existence in time requires anchoring that awareness to an external context, as independent verification through experience is paramount.