成唯識論述記 3.112

3.112

述曰：初舉三法喻，喻三法體；後舉二法喻，喻因果義，種、現二義其文可解。何故無有四法新種生現耶？心等一時，不可並故，又生彼緣未和合故。若許生者，便無窮故，不可此時遂生現行。

The Commentary states: First, the analogy of three dharmas is adduced to illustrate the substance of the three dharmas; next, the analogy of two dharmas is adduced to illustrate the meaning of cause and effect. The two meanings of seed and active manifestation can be understood from the text. Why are there not four dharmas, with the new seed producing an active manifestation? Because minds and so forth at a single moment cannot be concurrent, and furthermore the conditions for producing that have not yet harmoniously combined. If producing were admitted, there would be infinite regress, so at this time it cannot immediately produce an active manifestation.