I am proposing the following typology of Buddhist idealism.
(a) Ontological/metaphysical/objective idealism: There are no external phenomena; that is, extra-mental objects (bāhyārtha: phyi don) do not exist. Even as we seem to perceive extra-mental objects, such objects do not exist. In a dream, we might “perceive” a boulder, but such an object has never been there. Tibetans called it phyi don med pa’i sems tsam. I wonder if we can call such an idealism “synchronic idealism.” This proposition can be attributed to Cittamātravādins. According to some scholars, Vasubandhu in his Viṃśikā is said to propose also ontological idealism.
(b) Causal or diachronic idealism: There are external phenomena, but these are ultimately caused by or rooted in mind. Such an idealism negates any metaphysical prima causa (be it theistic or non-theistic). Some Tibetan sources seem to describe it as byed pa po gzhan med pa'i sems tsam. This should be acceptable to Buddhism in general, including scholars such as Bhāviveka and Candrakīrti. I wonder if I should call it causal or diachronic idealism. I recall Mi pham stating in his bsDus tshan something to this effect: the position that “matter arises out of mind” is Buddhist, whereas the position that “mind arises out of matter” is materialistic. The latter position (“materialism”) is suggestive of epiphenomenalism. The former would qualify as causal or diachronic idealism.
(c) Epistemological/subjective/representational idealism: We cannot directly perceive external phenomena, and all that we know is our own perception (or act of making known). This is expressed by the term vijñaptimātra(tā). Mi pham would put it thus: sems la snang na sems yin pas khyab. This should then be epistemological/subjective idealism, or perhaps representational idealism. This is perhaps the kind of idealism that Vasubandhu and Dharmakīrti proposed, although the nuances and implications of their positions need not be seen as identical. According to some scholars, Dharmakīrti proposes an epistemological/representational idealism (e.g., by means of sahopalambhaniyama reasoning) while remaining agnostic regarding ontological idealism. Vasubandhu in his Viṃśikā, on the other hand, starts off with epistemological/representational idealism and moves on to ontological idealism.
(d) Solipsistic idealism: Most Buddhist philosophers would reject a kind of idealism implying solipsism. Dharmakīrti’s Santānāntarasiddhi shows how seriously this objection was taken. Ratnakīrti may be an exception. He is fascinating and controversial. His arguments sometimes seem to push idealism toward solipsism, though scholars debate whether he really endorsed this or was exploring it dialectically. His position might be: “If you push vijñaptimātra(tā) to its logical extreme, you get something uncomfortably close to solipsism—so perhaps we need to reconsider the premises.” At minimum, his work shows that solipsism was recognized as a potential reductio ad absurdum of certain forms of idealism.
(e) Classificatory question: The question remains as to which type of idealism one should use to categorize the kind suggested by Yuktiṣaṣṭikā 35, which states that the four gross elements (mahābhūta) are contained in vijñāna. Does this mean type (a): the gross elements do not exist externally, only as contents of vijñāna? Does it mean type (b): the gross elements arise from or are caused by vijñāna? Or does it mean type (c): we only know the elements as they appear in vijñāna?