For Kant, space and time are a priori forms of intuition (Anschauung), meaning they are not derived from experience but are necessary conditions for experience itself. They structure all possible appearances, shaping how phenomena can be presented to us.

• Space is the form of outer sense, meaning it structures how we intuit objects as external to us. Without space, we could not perceive objects as outside one another or as having shape, distance, or extension.
• Time is the form of inner sense, meaning it structures how we experience our own mental states as succeeding one another. Without time, we could not have any awareness of change, succession, or simultaneity.

However, Kant does not frame space as merely "external" and time as merely "internal" in an absolute sense. Space structures external appearances, but it is still a form of our intuition. Likewise, time structures inner experience, but it also conditions external appearances, since any experience necessarily unfolds in time.