The matter is as follows: There are two levels, hishtalshelus and a level that transcends hishtalshelus. Hishtalshelus is the progressive chain of descent from one level to the next. It is similar to a shalsheles, or chain, which is composed of numerous linked rings. The upper part of each lower ring rests on the ring immediately above it, while the lower part of every upper ring is linked to the upper part of the ring immediately below it. Though each ring is linked to the next, each succeeding ring becomes further removed from the top one.
The same is true regarding knowledge. There is higher and lower wisdom. This is so both with regard to the various branches of knowledge and with regard to the various levels within each branch. There are different categories of knowledge, such as the natural sciences of botany, medicine, etc. There are also categories of knowledge which are supernatural, and then there is the knowledge of Torah and G-dliness.
In each of these subjects there is a higher and a lower. Yet in general, those branches of wisdom dealing with natural matters are lower than the branches dealing with the supernatural. The latter are, in turn, lower than the knowledge of Torah and G-dliness. For all other fields of knowledge are in the realm of human intellect, while Torah deals with G-dly intellect.
Though Torah also descends to the level of human comprehension, for the mitzvah of learning Torah can be performed only when Torah is understood, the essential concepts of Torah are G-dly. To gain G-dly wisdom involves comprehending G-dliness and G-d. G-dliness refers to Divine emanations, which are illumination but not essence. For example, the sun's light is only an emanation from the sun, while the sun itself is essence. For this reason the sun is called "ma'or," or "source of light," while the light which radiates from it is called "ha'orah," or "emanation."
The manner of understanding G-dliness is twofold: that G-d is all and that all is G-d. Both these aspects contain profound intellectual concepts. G-d Himself, however, is "atzmi," or "essence," and totally beyond comprehension. All that can be comprehended is ha'orah, the radiation emanating from His essence. Essence itself is far removed from emanation, as they are completely different from one another. However, this too can be apprehended, albeit to a limited degree, for in the gaining of wisdom there is an ordered progression of lower and higher, smaller and greater.
In summary: Knowledge is divided into various categories, some higher and some lower, similar to a chain in which every link bears some similarity to every other. This is hishtalshelus. The wisdom of Torah and G-dliness, though humanly comprehensible, transcends hishtalshelus. G-dliness in relation to G-d is similar to the rays of the sun compared to the sun itself. G-d is all; all is G-d.