Ishta Purtam is a compound word used in Rig Veda and Upanishads to denote the performance of sacrifices and other rituals ordained in the texts. Ishta is the results of yajna and rituals which are not visible but have to be believed in (ishta). Purta consists of acts useful to the public like digging tanks and wells, feeding the poor and doing other acts of charity (purta) helpful to society.
According to the doctrine of transmigration developed in the Upanishadic period, there are two distinct stages. The former is the earlier Vedic view of recompense in the next world. The later theory relates to the path of knowledge and meditation leading to Brahman (the supreme truth). The former is the way of pitra-yana (the forefathers), while latter is the way of deva-yana (the gods).
Ishta Purtam In Chandogya Upanishad
Chandogya Upanishad (V.10) states that those who perform charitable deeds or undertake such public works as the digging of wells follow after death the way of the fathers. That is, the soul after death enters first into smoke, then into night, the dark of the month, etc., and at last reaches the moon; after a residence there, as long as the remnant of the good deeds remain, it descends again through ether, wind, smoke, mist, cloud, rain, plants, food and seeds. Then, through the assimilation of food by man, it is said to enter the womb of a mother and be born again.
In Mundaka Upanishad, the superiority of the path of knowledge over the path of action is clearly brought out (II.10)
“The ignorant people, thinking sacrificial and charitable acts as most important, do not know any other way to bliss. Having enjoyed themselves in the highest heaven, the abode of pleasure, they enter again into this or inferior worlds” (Mundaka Upanishad 2.10)
Addressing the householder, who is under the impression that by performing rituals ordained in Vedas and doing service to humanity one can reach the highest place of happiness, it points out that such results are only transient and that real happiness can come only from true knowledge.
Though ordinary people may be satisfied with the results obtained from these two types of work, the wise man intends to go beyond and work for humanity without any expectation of rewards, according to the texts.
The good results such as the attainment of heaven or happiness in the next life do not attract the wise man, who is after true knowledge, which leads to true liberation.
Source – Encyclopedia of Hinduism Volume V page 194