大乘起信論義記 3.244

3.244

標中言推求五陰色之與心者，色陰是色，餘四是心。釋中先觀色法。言六塵無念者，境從心起畢竟無體，離心之外無可念相也。言以心無形求不得者，非直心外無別色等六塵，就心內求色等形質亦不可得也。前則所緣無相，此則能緣不生也。

In stating it, the phrase 'investigates the five skandhas—matter and mind' means that the form skandha is matter, and the remaining four are mind. In the explanation, one first contemplates physical phenomena. The phrase 'the six dusts are without thought' means that objects arise from the mind and are ultimately devoid of substance; outside of the mind there are no marks that can be thought of. The phrase 'because the mind has no shape, seeking it, it is unobtainable' means not only that there is no separate matter or other six dusts outside of the mind, but also that seeking physical entities such as matter within the mind is likewise impossible. The former means that the perceived object is markless; the latter means that the perceiving subject does not arise.