Who are Adi Shakti and Durga?
Goddess Durga herself is Adi Parashakti. In Sri Chandi (from the Markandeya Purana), she is described as the combined form of the 3 deities viz; Mahakali, Mahalakshmi and Mahasaraswati.
The Devi Atharva Sirsha describes Adi Parashakti's features as follows:
sābravīt- ahaṃ brahmasvarūpiṇī ।
mattaḥ prakṛtipuruṣātmakaṃ jagat ।
śūnyaṃ cāśūnyam ca ॥2॥
She said: I am an aspect of Brahma. From me this Universe, in form of Prakriti and Purusha, is generated; which is both void and non-void.
ahaṃ vijñānāvijñāne ।
ahaṃ brahmābrahmaṇī veditavye ।
ahaṃ pañcabhūtānyapañcabhūtāni ।
ahamakhilaṃ jagat ॥3॥
I am both bliss and non-bliss. I am knowledge and non-knowledge. I am Brahma and non-Brahma (the non-manifest state called Abrahma). I am the five primordial principles and non-principles. I am the whole perceived Universe.
vedo ̕hamavedo ̕ham।
adhaścordhvaṃ ca tiryakcāham ॥4॥
I am Veda (knowledge about Brahma) and non-knowledge. I am learning and ignorance. I am unborn and also born. I am up, down and in the middle.
ahaṃ rudrebhirvasubhiścarāmi ।
ahamādityairuta viśvadevaiḥ ।
ahaṃ mitrāvaruṇāvubhau bibharmi ।
ahamindrāgnī ahamaśvināvubhau ॥5॥
I move about in form of Rudra and Vasu. I move about as Aditya and all the Gods. I sustain Mitra, Varuna, Indra, Agni and both the Ashvinas.
ahaṃ somaṃ tvaṣṭāraṃ pūṣaṇaṃ bhagaṃ dadhāmi।
ahaṃ viṣṇumurukramaṃ brahmāṇamuta prajāpatiṃ dadhāmi ॥6॥
In me there are Soma, Tvasta, Pusha and Bhaga. In me there are Vishnu with wide footsteps, Brahma and Prajapati.
saiṣā dvādaśādityāḥ ।
saiṣā viśvedevāḥ somapā asomapāśca ।
saiṣā yātudhānā asurā rakṣāṃsi piśācā yakṣāḥ siddhāḥ । saiṣā sattvarajastamāṃsi ।
saiṣā prajāpatīndramanavaḥ ।
saiṣā grahanakṣatrajyotīṃṣi ।
tāmahaṃ praṇaumi nityam |
pāpahāriṇīṃ devīṃ bhuktimuktipradāyinīm ।
anantāṃ vijayāṃ śuddhāṃ śaraṇyāṃ śivadāṃ śivām॥17॥
She is the eight Vasus. She is the eleven Rudras. She is the twelve Adityas. She is the all the Gods, drinking Soma or non-drinking. She is Yatudhan, Rakshashas, Asuras, Pishachas, Yakshas and Siddhas (kinds of demonic or malicious beings). She is Satva, Rajas and Tamas. She is Brahma, Vishnu and Rudra. She is Prajapati, Indra and Manu. She is the planets, stars and constellations. She is various measures of Time. She is destroyer of sins, giver of both enjoyment and Moksha, without end, giving victory over distractions of Samsara, pure, worth taking refuge, giver of peace, peace personified, we pray to Her.
And, then finally it says, that she is Durga only or known by that name.
yasyāḥ svarūpaṃ brahmādayo na jānanti tasmāducyate ajñeyā ।
yasyā anto na labhyate tasmāducyate anantā ।
yasyā lakṣyaṃ nopalakṣyate tasmāducyate alakṣyā ।
yasyā jananaṃ nopalabhyate tasmāducyate ajā ।
ekaiva sarvatra vartate tasmāducyate ekā ।
ekaiva viśvarūpiṇī tasmāducyate naikā ।
ata evocyate ajñeyānantālakṣyājaikā naiketi ॥23॥
Even Brahma and the other Devas do not know her real form, so she is called Ajñeya. We do not find its limit, so she is called Ananta. We can not find the meaning, so she is called Alakshya. Her birth is not known, so she is called Aja. She is found everywhere, so she is called Eka, the One. She has taken up all the various forms, so she is called Naika. Because of this she is called these various names.
mantrāṇāṃ mātṛkā devī śabdānāṃ jñānarūpiṇī ।
jñānānāṃ cinmayātītā śūnyānāṃ śūnyasākṣiṇī ।
yasyāḥ parataraṃ nāsti saiṣā durgā prakīrtitā ॥24॥
Amongst the mantras She is the original sounds, in the words She is the essence of knowledge. Amongst the knowledge She is beyond what is comprehensible by analysis and by deep meditation, She is the witness to that state. She is well known as Durga, there is nothing better than her.
tāṃ durgāṃ durgamāṃ devīṃ durācāravighātinīm ।
namāmi bhavabhīto ̕haṃ saṃsārārṇavatāriṇīm ॥25॥
I, afraid of the Samsara, offer my salutations to Durga, difficult to know, destroyer of sins and pilot while crossing this sea of Samsara.
And so is Goddess Parvati, because as per the Puranas, Lord Shiva did some severe penance and that's how obtained Adi Parashakti as his consort. This story i have mentioned in some other answer of mine.
So, that's why Goddess Durga, Goddess Parvati are only said to be Adi Parashakti.
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