As stated in the Kaṭha Upaniṣad (2.3.9, 12):

na sandṛśe tiṣṭhati rūpam asya
 na cakṣuṣā paśyati kaścanainam
hṛdā manīṣā manasābhikḷpto
 ya etad vidur amṛtās te bhavanti

naiva vācā na manasā prāptuṁ śakyo na cakṣuṣā

“Spirit is not within the jurisdiction of material eyes, words or mind.”

Similarly, Śrīmad-Bhāgavatam (10.84.13) states:

yasyātma-buddhiḥ kuṇape tri-dhātuke
 sva-dhīḥ kalatrādiṣu bhauma ijya-dhīḥ
yat-tīrtha-buddhiḥ salile na karhicij
 janeṣv abhijñeṣu sa eva go-kharaḥ

“A human being who identifies his body made of three elements with his self, who considers the by-products of his body to be his kinsmen, who considers the land of his birth worshipable, and who goes to a place of pilgrimage simply to take a bath rather than to meet men of transcendental knowledge there is to be considered like an ass or a cow.”

These are some Vedic statements about spiritual substance. Spiritual substance cannot be seen by the unintelligent, because they do not have the eyes or the mentality to see the spirit soul. Consequently they think that there is no such thing as spirit. But the followers of the Vedic injunctions take their information from Vedic statements, such as the verses from the Kaṭha Upaniṣad and Śrīmad-Bhāgavatam quoted above..