As has been indicated...

As has been indicated, the most certain of all the kinds of qualities are psychic qualities with which one becomes acquainted through knowledge by presence and inner experience. Even the likes of Hume, who has raised doubts about many certainties, has considered the existence of this group of qualities to be certain and undeniable.

Among the types of psychic quality, that which has the greatest relevance to philosophical discussions is knowledge, and for this reason there will be an independent discussion of this. After knowledge, will, power, and freedom are considered, which were discussed in Lesson Thirty-Eight, and more explanations pertaining to them will be found in the discussions of the attributes of God Almighty.

Sensible Qualities By sensible qualities are meant those material qualities which are perceived through the external senses and sensory organs.

On the basis of a view which was accepted in ancient natural science, according to which the external senses are of five kinds, philosophers have divided the sensory qualities into five groups: color and light as visible qualities, sounds as audible qualities, tastes as gustatory qualities, smells as olfactory qualities, and cold, hot, rough and soft as tactile qualities.

But in modern psychology, it has been proven that there are other senses in addition to the five well-known senses which must be taken into consideration when classifying the sensory qualities. The proof of the existence of sensible qualities outside the realm of perception is not as easy as proving psychic qualities, for knowledge by presence does not apply to them.

The question may be raised as to whether what we perceive as states of material things exist in the same way in the context of the external world, or whether the soul is capable of perceiving these things within itself as a result of a chain of physical, chemical and physiological actions and reactions, while they themselves cannot be proven to exist in the material world.

In order to provide a correct answer to this question one must make use of arguments whose premises are drawn from the empirical sciences. The definitive establishment of these sorts of premises depends on the progress of the relevant sciences. For example, the whatness of energy and the relation between matter and energy are not yet known with certainty, and for this reason a definitive philosophical analysis cannot be provided for them.