Q: What are the twenty-five Tattvas of Samkhya?
A: The principles or Tattvas are twenty-five in number. They are five gross elements (Bhutas), viz.
(1) Ksiti, (2) Ap, (3) Tejas, (4) Vayu, and (5) Akasa characterized respectively by smell, taste, visual form, thermal sensation and sound;
the five subtle monads (Tanmatras), viz.
(6) smell (Gandha), (7) taste (Rasa), (8) light (Rupa), (9) thermal (Sparsa) and (10) sound (Sabda);
the five organs of action, viz.
(11) vocal organ (Vak), (12) manual organ (Pani), (13) organ of locomotion (Pada), (14) excremental organ (Payu) and (15) genital organ (Upastha);
the five sense-organs, viz.
(16) the auditory sense (Karna), (17) the thermal sense (Tvak), (18) the visual sense (chaksu), (19) the gustatory sense (Jihva) and (20) the olfactory sense (Nasa);
(21) mind (Manas) where thoughts and determinations originate,
(22) the mutative ego or I-sense (Ahamkara or Asmita) which sustains feelings like ‘I am this or that’ and also kindles one’s possessive instincts like ‘this is mine’,
(23) the pure “I”-sense (Buddhi-tattva) is pure I-sense bereft of all attributes and
(24) Prakrti or Pradhana or the three Gunas (Sattva, Rajas, Tamas) in unmanifest state. All knowables converge in them. It forms the base material constituent and is the cause of all the above twenty-three principles (upto Buddhi-tattva).
These twenty-four principles and beyond them the absolute Knower or (25) Purusa, the efficient cause of all phenomena, make up the twenty-five principles or Tattvas of the samkhyaites… (taken from Tattvas and their realization article of the book Yoga Philosophy of Patanjali with Bhasvati by Swami Hariharananda Aranya)