Pujya Swamiji explained the verses 17&18.
The external world is material or Bhouthikam (Panchabhūta kruta) in nature. The Pancha Bhūtas (Ākāsha, Vayu, agni, jalam&prithvi) have five gunas (shabda, sparsha, rupa, rasa&gandha) and the world also has the five gunas. It is very clear that external, gross world is the effect of five elements, as we can perceive it. We can know that physical body is also matter, as we can perceive it.
How one can understand the Mind and the ten indriyas are made up of five elements, as they are not perceived like the worldly objects or the physical body?
Pujya Vidyaranyaji explains that with the help of logic (yukti or reasoning)&Sastra (trustworthy source, Āpta Vākyam), it can be known that the mind and ten indriyas are also made up of matter.
1.The Chandogyopanishad (ch 6) is cited for Saastra pramaanam. It is explained in Chandogyopanishad as follows.
Annamayam hi Soumya Manaha.
The mind is made up of food. The food we take is divided into three parts. The grossest part is excreted out as feces. The middle sūkshma part becomes energy and the subtlest part becomes mind.
The Prāna are made up of water. (Gross aspect of water we take becomes urine, middle part becomes blood etc. and the subtlest part becomes Prāna).
The speech is made up of fire.
An experiment done by Uddālaka Rishi (father) on his son Svetaketu explains the fact that mind is made up of food. He asks his son not to eat food for fifteen days and asks him to take plenty of water. When the son comes back after fifteen days to see his father, the father asks him to repeat a portion of Rig-Veda (which the son already learnt). But, the son is not able to repeat the vedam asked by his father as the mind is weakened by the lack of food. The mind did not function. He is able to retain his Prāna (the Prāna is functioning) as he is taking lot of water.
It can be analyzed by Anvaya vyatireka logic. The logic of co presence and co absence. The absence of food makes the mind weaker and vice versa.(Food is the nourisher of the mind).The mind depends on the five elements. In this way, it has been proved that the mind is made up of five elements.
2. There is the connection between the elements and the organs. The Auditory organ (srotra) being made up of Ākāsha can comprehend sound which is the property of Ākāsha.
Tattad Indriyam tattad Bhūta janyam tattad Bhūta guna grāhakatvāt.
Indriya born from that element can comprehend the property of that element from which it is born.
3.Kārana Gunān Kāryeshu Prārabhante.
The properties belonging to the cause will enable the properties in their effects.
Properties of the effects are directly proportional to their causes.
If not for Vāyu being the cause of Tvak, Tvak (skin) will not be able to comprehend touch and the same rule applies to other elements also.
In this way, the author proves that the mind and the ten senses are made up of elements and bhouthikam only.
The 18th verse is explained as follows.
The word “Idam” or this denotes whatever we experience through the eleven sense organs.(ten senses and the mind).
The entire objects of the world acted upon by karmendriyas, comprehended by jnanendriyas and all the three mental modifications comes under the word Idam or this.
Whatever knowledge (however little) we understand through the means of knowledge comes under Idam or this.
There are six means of knowledge.
Pratyaksha or Perception.
Anumāna or Inference.
Upamāna or Comparison.
Arthāpatti or Postulation.
Anupalabhdhi or Non-comprehension.
Shabda or words from a trust worthy source (sāstram or Āpta vākya or from a trustworthy person).
All the five elements, Jnanendriyas, Karmendriyas, Antahkaranam and all the fourteen lokās and all the things that can be known by logic and all the knowledge obtained by the six means of knowledge comes under the word,”Idam or this”.
This is the Idam word in the sentence (chandogyaopanishad),” “Sad eva, soumya, idam agra asid ekam evadvitiyam”.