Prabhupāda: What is modality?
Śyāmasundara: Modality means whether it is possible or impossible; whether it is existent or nonexistent; whether it is necessary or dependent. Like that.
Prabhupāda: That's all right.
Śyāmasundara: So this is the process of material reasoning, that when we see something, we can understand it by applying our reason, that it is such-and-such weight, it is measured with the mind. This is called the categorical imperative. The next, third step, is called the transcendental dialectic. In this stage, beyond reasoning, the mind seeks to understand everything, but the sense information is inadequate, so it tries to go beyond sense experience.
Prabhupāda: How?
Śyāmasundara: He says the mind is aware that there is an ultimate reality, or a thing in itself, a noumenon, which produces each phenomenon, but the mind is not equipped to sense this ultimate reality. So the mind must remain forever content to be agnostic.
Prabhupāda: No. He should go to higher authorities. Why should he remain agnostic? If there is possibility, mind cannot go beyond this, but if the same thing, we say upon the roof there is some sound, now we speculate, but we cannot ascertain what is the sound. But if somebody is actually there, he says, "This sound is due to this." So why I shall remain satisfied with agnostic position, that I could not ascertain what is the sound, and therefore I shall remain satisfied? I shall say, "Is there anybody on the roof?" If somebody says, "Yes. I am here," "Will you kindly say what is the sound?" "Yes: this, that, this, that." Therefore Vedic injunction is tad vijñānārtham: that which is beyond your senses, you must approach a spiritual master. He will give you information. That is our system, accepting guru. Tasmād guruṁ prapadyeta jijñāsuḥ śreya uttamam (SB 11.3.21). One who is inquisitive to understand the transcendental subject, he must approach a guru. What is guru? Śābde pare ca niṣṇātam: guru, who is expert or well versed in the Vedic literatures, śruti. And what is the result? How can I understand that he is well versed in Vedic literature? Brahmaṇy upaśamāśrayaḥ. He has forgotten everything material; he is simply concerned with the spirit soul. That's all. Everything is there. So Kant here is imperfect in his knowledge.