The quality of the sun is brightness. The quality of water is coolness. The quality of fire is heat. In the same way, the quality of the atman or I, is existence. This existence is in the form of knowledge. Knowledge is in the form of happiness. The three qualities of the atman are Existence, Knowledge and Bliss. They are not three different qualities, but are known by these names.
Sri Adi Sankaracharya says that the intellect is not I. The intellect is an instrument of knowledge. The body is like the pen. The mind is the nib. The intellect is like the tip of the nib and the atman is like the ink. How much ever important the pen, the nib and its tip may be, it is because of the ink that the writing happens. Knowledge and utter bliss of experience is the very characteristic of the atman. It is pure and eternal.
Everything in nature expresses itself the way it is. It cannot be changed. One fine day, you cannot have naturally hot water flowing from the tap. You cannot have a sun that has no light. You cannot strike a match stick and experience fire that is very cool. Objects of nature can only express their true qualities. In the same way, the atman expresses itself only the way it is as the eternal and unsullied existence.
There is the quality of existence and the power of knowing, consciousness in the self or atman. In the intellect, there is a movement called thought. When this light of consciousness falls on the thought, there is an immediate recognition in the intellect called ‘I know’. The I belongs to the atman and the act of knowing belongs to the instrument of knowledge called the intellect. Yet, the intellect superimposes the knowing thought on the I thought. They merge in an indistinguishable way and this thought called ‘I know’ is born.
On the ocean’s surface there is illumination of sunlight. It highlights the ocean. On each particular wave, the sunlight falls in the shape of a wave and makes it look distinctly different from the light that is all over the ocean surface. In the same way, there is a general consciousness that illumines the intellect. When thought waves surface, the consciousness gets specially reflected on the thoughts.
The word I is unfortunately added to every experience of ours. Consciousness is reflected generally in the intellect and that reflection is flashed on thoughts, feelings, emotions and ideas and even on the functions of the body.