In Max Muller's translation of the Critique of Pure Reason, Kant states, in "Transcendental exposition of the concept of space", that:
Space must originally be an intuition; for from a mere concept it is impossible to obtain propositions which go beyond the concept, such as we do, however, be encountered in us a priori, that is prior to any perception of an object, and must therefore be pure, not empirical, intuition.
I understand the reasoning behind why space is a pure and not an empirical intuition. However, I do not understand why Kant wrote that space must be an intuition and not a concept; is it not possible to obtain synthetic a priori propositions which go beyond the concept as argued in Part V of Introduction B? Or does the process of obtaining a proposition, in this context, differ from what had been mentioned in the introduction?
Disclaimer: I am a high school student attempting to understand Kant's Critique of Pure Reason, supported by my independent research, so I may misconstrue some of Kant's ideas and I apologize for that. If possible, please try to tailor your responses to a reader with a weaker background and poorer grasp of philosophical concepts. If I come across as a dilettante, I am sorry for wasting your time in reading the post.