The reason there are two arguments is because of the Buddha’s investigation method, which is to examine both the external or synthesis, and internal or analysis. This means looking at how a thing operates within its environment and also how it is constructed. In general the Buddha was known in those times as a discriminating teacher (AN 10.94). In practice in the suttas the often used method of realization of non-self is through cultivating knowledge of impermanence. The way to approach contemplation of impermanence is to investigate the illusion of continuity exhibited by all material things. Reflection on their hidden life of ageing, decay, and death with concrete examples will eventually disclose the fabricated nature of perception. This meets internal resistance. As Analayo says, this is the ‘power’ aspect of meditation upon which everything else relies. It was this that enabled the awakening of the five companions to stream-entry when the Buddha delivered the present second discourse.
SuttaCentral