“However, you will have to bear in mind (a) that we recognize but one element in Nature (whether spiritual or physical) outside which there can be no Nature since it is Nature itself, and which as the Akasa pervades our solar system, every atom being part of itself, pervades throughout space and is space in fact, . . . (b) that consequently spirit and matter are one, being but a differentiation of states not essences, . . . (c) that our notions of “cosmic matter” are diametrically opposed to those of western science. Perchance if you remember all this we will succeed in imparting to you at least the elementary axioms of our esoteric philosophy more correctly than heretofore.” (The Mahatma Letters, letter #11, 3rd ed., p. 63; chron. ed. letter #65, p. 168).

“Yes, as described in my letter—there is but one element and it is impossible to comprehend our system before a correct conception of it is firmly fixed in one’s mind. You must therefore pardon me if I dwell on the subject longer than really seems necessary. But unless this great primary fact is firmly grasped the rest will appear unintelligible. This element then is the—to speak metaphysically—one sub-stratum or permanent cause of all manifestations in the phenomenal universe.” (The Mahatma Letters, letter #15, 3rd ed., p. 89; chron. ed. letter #67, p. 182).

The one element, Sanskrit eka-dhātu, Tibetan khams gcig.

From the commentary on Ratnagotravibhāga, chapter 1, verse 12, E. H. Johnston edition, p. 13, followed by my English translation:

All because they do not know and do not see the one element.

vibandhaḥ punar abhūta-vastu-nimittârambaṇa-manasikāra-pūrvikā rāga-dveṣa-mohôtpattir anuśaya-paryutthāna-yogāt | anuśayato hi bālānām abhūtam atat-svabhāvaṃ vastu śubhâkāreṇa vā nimittaṃ bhavati rāgôtpattitaḥ | pratighâkāreṇa vā dveṣôtpattitaḥ | avidyâkāreṇa vā mohôtpattitaḥ | tac ca rāga-dveṣa-moha-nimittam ayathā-bhutam ārambaṇaṃ kurvatām ayoniśo-manasikāraś cittaṃ paryādadāti | teṣām ayoniśo-manasikāra-paryavasthita-cetasāṃ rāga-dveṣa-mohānām anyatama-kleśa-samudācāro bhavati | te tato nidānaṃ kāyena vācā manasā rāga-jam api karmâbhisaṃskurvanti | dveṣa-jam api moha-jam api karmâbhisaṃskurvanti | karmataś ca punar-janmânubandha eva bhavati | evam eṣāṃ bālānām anuśayavatāṃ nimitta-grāhiṇām ārambaṇa-caritānām ayoniśo-manasikāra-samudācārāt kleśa-samudayaḥ | kleśa-samudāyāt karma-samudayaḥ | karma-samudayāj janma-samudayo bhavati | sa punar eṣa sarvâkāra-kleśa-karma-janma-saṃkleśo bālānām ekasya dhātor yathā-bhūtam ajñānād adarśanāc ca pravartate |

“The obstruction [to seeing reality, tattva] is the arising of attraction, aversion, and delusion [the three main “afflictions,” or defilements, kleśa], due to the activation of a latent tendency toward the defilements, preceded by mental engagement or taking to mind as an object of thought the outward appearance of a thing, not as it really is. For, due to a latent tendency toward the defilements on the part of the spiritually immature, a thing not as it really is, i.e., not having the inherent nature of that thing, becomes an outward appearance in the form of something beautiful, due to the arising of attraction. Or in the form of something repugnant, due to the arising of aversion. Or in the form of something wrongly known, due to the arising of delusion. And for those taking as an object of thought that outward appearance resulting from attraction, aversion, and delusion, not as the thing really is, the incorrect mental engagement takes over the mind. For those whose minds are taken over by incorrect mental engagement, there comes about the manifestation or operation of any one of the defilements: attraction, aversion, or delusion. Because of that, they perform actions by body, speech, and mind, born from attraction. Also born from aversion, also born from delusion, they perform actions. And due to actions, there certainly comes about the succession of rebirths. In this way for these spiritually immature people, who have latent tendencies toward the defilements, who apprehend outward appearances, who go with them as objects of thought, due to the operation of incorrect mental engagement the defilements arise. Due to the arising of defilements, actions arise. Due to the arising of actions, (re-)births arise. This very defilement by all kinds of defilements, actions, and (re-)births occurs for the spiritually immature because they do not know and do not see the one element as it really is.”