Without more context, it is difficult to provide a specific analogy used by the author to explain why they will not write about slavery with limitation. However, one possible analogy that is commonly used to explain the importance of not limiting discussions of slavery is that it is like trying to discuss a cake recipe without mentioning flour. Just as flour is a fundamental and essential ingredient in making a cake, so too is slavery a fundamental and essential aspect of understanding the history and legacy of many societies. To limit discussions of slavery is to ignore a key ingredient in the making of a society, and ultimately, to distort our understanding of history.