Devotees would often ask Sri Ramakrishna to explain the great spiritual truths since they knew that he had the highest spiritual realization and had attained the highest knowledge of Brahman. In reply Sri Ramakrishna would say, “What Brahman is cannot he described. All things in the world - the Vedas, the Puranas, the Tantras, the six systems of philosophy - have been defiled, like food that has been touched by the tongue, for they have been read or uttered by the tongue. Only one thing has not been defiled in this way, and that is Brahman. No one has ever been able to say what Brahman is.” He would further elaborate the idea using a beautiful story.
A man had two sons. The father sent them to a preceptor to learn the Knowledge of Brahman. After a few years they returned from their preceptor’s house and bowed low before their father. Wanting to measure the depth of their knowledge of Brahman, he first questioned the older of the two boys. ‘My child,’ he said, ‘You have studied all the scriptures. Now tell me, what is the nature of Brahman?’ The boy began to explain Brahman by reciting various texts from the Vedas. The father did not say anything. Then he asked the younger son the same question. But the boy remained silent and stood with eyes cast down. No word escaped his lips. The father was pleased and said to him: ‘My child, you have understood a little of Brahman. What It is cannot be expressed in words.’
Thus Sri Ramakrishna would say that Brahman was beyond expression. That is why the scriptures speak of Brahman as unattainable by speech and even mind. The reason is mind, speech, etc belong to the realm of the senses whereas Brahman stands for the highest transcendental truth far beyond the sense world. However, it can be realized by true spiritual seekers through intense Sadhana.