The antonyms for the word "in-substantial" include "substantial," "solid," "massive," and "weighty." All of these words convey a sense of materiality and heft that is lacking in the word "in-substantial." "Substantial" in particular implies a real or tangible quality, while "solid" suggests a cohesion and durability that is missing in something that is in-substantial. "Massive" and "weighty" point to the physical size or weight of an object, its gravity and presence, qualities that are completely absent in something that is in-substantial. These antonyms serve to highlight the deficiencies of something that is in-substantial and show us the importance of having a real, material foundation in our lives.