Sankaracarya's Vivekachudamani 476

स्वस्याविद्याबन्धसंबन्धमोक्षात्

सत्यज्ञानानन्दरूपात्मलब्धौ ।

शास्त्रं युक्तिर्देशिकोक्तिः प्रमाणं

चान्तःसिद्धा स्वानुभूतिः प्रमाणम् ॥

svasyāvidyābandhasaṃbandhamokṣāt

satyajñānānandarūpātmalabdhau |

śāstraṃ yuktirdeśikoktiḥ pramāṇaṃ

cāntaḥsiddhā svānubhūtiḥ pramāṇam ||

(Sankaracarya's Vivekachudamani 476)

When the Self, the Existence-Knowledge-Bliss, is realized, through liberation from one's bondage of ignorance, then the scriptures, logical reasoning, the words of the Teacher--these are proofs; the internal realization of one's own concentrated mind is yet another proof.

When one is ignorantly attached to the material world and other bondage in this world, he/she cannot understand the logical reasons in the Vedas. The ignorant person will blindly follow his senses or mind. Unless one understands the existence of the soul and its blissfulness then he should materially be exhausted to understand the Paramatma and soul. Therefore a spiritual master is needed to impart our spiritual knowledge without any deviation. Those who are adamant have to take repetitive birth and suffer if they do not take this human life as valuable.