大乘起信論義記 3.244

3.244

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.