The phrase "in substantiality" can be replaced with a variety of synonyms. Some options include "in essence," "fundamentally," "principally," "essentially," and "at its core." These words all imply that something is being described in a way that emphasizes its most important or foundational features. Synonyms for "in substantiality" can also include "in reality," "in truth," or "actually." These words suggest that the speaker is attempting to get to the heart of the matter, to describe something in a way that is undeniably true or factual. In general, synonyms for "in substantiality" all point to the underlying nature of something, rather than its superficial or less important details.