首楞嚴義疏注經 1.163

1.163

下文識精元明，即是無始菩提涅槃元清淨體。此體非妄，無有變異，故云真性。研，究窮也。《淨名》云「直心是道場，無虛假故。」今推本意，豈得異想發言。欲正修行，當須確實，故今勸也。

The 'fundamental clarity of the essence of consciousness' mentioned below is precisely the beginningless, fundamentally pure essence of bodhi and nirvāṇa. This essence is not deluded and undergoes no change; hence it is called the 'true nature.' 'Investigate' means to probe to the depths. The Vimalakīrtinirdeśa states: 'The straightforward mind is the bodhimaṇḍa, because it is free from falsehood.' Now, in investigating the root intention, how could one speak with aberrant thoughts? If one wishes to practice correctly, one must be genuine and real; hence the exhortation now.