Bahá’í Sacred Writings, ‘Abdu’l-Bahá, #314
There is a point that is pivotal to grasping the essence of the other questions that we have discussed or will be discussing, namely, that human knowledge is of two kinds.
One is the knowledge acquired through the senses. That which the eye, the ear, or the senses of smell, taste, or touch can perceive is called “sensible”. For example, the sun is sensible, as it can be seen. Likewise, sounds are sensible, as the ear can hear them; odours, as they can be inhaled and perceived by the sense of smell; foods, as the palate can perceive their sweetness, sourness, bitterness, or saltiness; heat and cold, as the sense of touch can perceive them. These are called sensible realities.
The other kind of human knowledge is that of intelligible things; that is, it consists of intelligible realities which have no outward form or place and which are not sensible. For example, the power of the mind is not sensible, nor are any of the human attributes: These are intelligible realities. Love, likewise, is an intelligible and not a sensible reality. For the ear does not hear these realities, the eye does not see them, the smell does not sense them, the taste does not detect them, the touch does not perceive them. Even the ether, the forces of which are said in natural philosophy to be heat, light, electricity, and magnetism, is an intelligible and not a sensible reality. Likewise, nature itself is an intelligible and not a sensible reality; the human spirit is an intelligible and not a sensible reality.
But when you undertake to express these intelligible realities, you have no recourse but to cast them in the mould of the sensible, for outwardly there is nothing beyond the sensible. Thus, when you wish to express the reality of the spirit and its conditions and degrees, you are obliged to describe them in terms of sensible things, since outwardly there exists nothing but the sensible. For example, grief and happiness are intelligible things, but when you wish to express these spiritual conditions you say, “My heart became heavy”, or “My heart was uplifted”, although one’s heart is not literally made heavy or lifted up. Rather, it is a spiritual or intelligible condition, the expression of which requires the use of sensible terms. Another example is when you say, “So-and-so has greatly advanced”, although he has remained in the same place, or “So-and-so has a high position”, whereas, like everyone else, he continues to walk upon the earth. This elevation and advancement are spiritual conditions and intelligible realities, but to express them you must use sensible terms, since outwardly there is nothing beyond the sensible.
To cite another example, knowledge is figuratively described as light, and ignorance as darkness. But reflect: Is knowledge sensible light or ignorance sensible darkness? Certainly not. These are only intelligible conditions, but when you wish to express them outwardly you call knowledge light and ignorance darkness and say, “My heart was dark and it became illumined.” Now, the light of knowledge and the darkness of ignorance are intelligible realities, not sensible ones, but when we seek to express them outwardly, we are obliged to give them a sensible form.