The Isâ Upanishad (or Ishavasyopanishad) happens to be one of the shortest yet deeply written ancient Hindu spiritual texts. For anyone willing to study Hinduism and engage in Hindu practices, this Upanishad provides an excellent point of entry because of the shortness and depth of its contents. 

Overview

The Isha Upanishad consists entirely of 18 verses (mantras). It also happens to be part of the Shukla Yajurveda, which is the most popular of the four Vedas. One cannot, however, deny that such few verses convey the core important principles of philosophy. By the way, "Isha's" definition corresponds to "Lord" or "Supreme Being", which hints at the consideration this text gives to ultimate reality. 

Central Themes.

Under the Isha Upanishad lies a number of interconnected themes:

1. Divine Immanence The first verse makes it clear that the whole universe is within the Divine:

"Ishavasyam idam sarvam, yat kincha jagatyam jagat" (Verse 1) "Everything, in whatever place, should superinduce the God on this changing universe."

This primary concept suggests that Divinity does not separate itself from creation; however, it holds unapproachable intimacy with interior objects.

2. In between abstinence and indulgence.   

The Upanishad teaches a middle path, which does not deny life in the world, nor does it indulge inworldly life in an indiscriminate manner: 

Tena tyaktena bhunjitha, ma gridhah kasya svid dhanam "Enjoy it [the world] in renouncing. Never covet anybody's wealth."  

Such a strangely paradoxical command leads us to practice the truth of living in the world outwardly, all the while maintaining inner detachment: the enjoyment of life without being owned by the possessions or desires.

3. Oneness in Succedaneous Contraries. 

The treatise is such that it brings about an apparent contradiction but points out the essential underlying unity in everything:  

"Tad ejati tan naijati tad dure tad v antike" "It moves and it moves not; It is far and It is near; It is within all this and It is outside of all this." 

Ultimate reality rises above our usual dualistic thinking.  

Harmony of Knowledge and Action 

These verses address the relationship of knowledge (vidya) and action (avidya), such that both are necessary for spiritual fulfillment: 

"Andham tamah pravishanti ye'vidyam upasate Tato bhuya iva te tamo ya u vidyayam ratah" "Into blind darkness enter those who worship ignorance (mere ritualistic works). Into greater darkness enter those who worship knowledge alone." 

Practical Application   

For practical principles that could be taken to heart in practicing Hinduism, the Isâ Upanishad has said the following: 

See the Divine in Everything: Recognize the sacred in all aspects of creation. 

Non-Possessiveness: Live with awareness that nothing really belongs to us. 

Balanced Living: accept neither the world nor its master. I

Integrative Spiritualism: Knowledge, devotion, and action go together. 

History and Influence  

In Isha Upanishad texts, these are ancient texts under the title Shruti (i.e. "that which is heard"), believed to be a direct revelation from God but not understood as penned by humans. It is the most important Upanishad (mukhya) that gives commentaries by great Hindu philosophers like Adi Shankara and Madhvacharya.

Tradition believes that thus made the Upanishads during the range of 800-500 BCE; thus, they are timeless (apaurusheya) in the reckoning of traditional Hindus. 

Further Resources on Study:   

For more in-depth studies consider: 

The Principal Upanishads by S. Radhakrishnan 

Eight Upanishads with the Commentary of Shankaracharya translated by Swami Gambhirananda 

The Upanishads: Breath of the Eternal by Swami Prabhavananda. 

The Ten Principal Upanishads by Shree Purohit Swami and W.B. Yeats.