Wikipedia
The Vaibhāṣika was an early Buddhist subschool formed by adherents of the Mahāvibhāṣa Śāstra, comprising the orthodox Kasmiri branch of the Sarvāstivāda school. The Vaibhāṣika-Sarvāstivāda, which had by far the most "comprehensive edifice of doctrinal systematics" of the early Buddhist schools, was widely influential in India and beyond.
The school was originally of a mystical nature, later developing into more materialistic concerns with a focus upon Materialism and 'existent phenomena' (Tibetan: yod-pa). The key tenets of this school are "that no mental concept can be formed except through direct contact between the mind, via the senses, such as sight, touch, taste, etc., and external objects".
Berzin (2007) elaborates this further:
Vaibhashika asserts sensory nonconceptual cognition of an object through direct contact with it, without the medium of a mental aspect of the object. Because of that, when something made of parts is validly known, the cognition must simultaneously also take as its objects the parts on which the object depends.