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In this article, we discuss semantic dementia (SD) as a clinical model of semantic system loss. We provide arguments in favor of heterogeneous semantic breakdown, regarding the nature of the knowledge assessed. We rely more specifically on two recent works by our team, highlighting that thematic relationships, organizing knowledge based on their complementarity within an event, have a particular status in SD. This knowledge appears to be residual, more robust to the disease, and may be over-activated in SD patients. In addition, these results support the hypothesis of a semantic disequilibrium between two types of semantic processing, namely taxonomic and thematic processing. This hypothesis states that the two types of semantic processing are kept in balance in healthy subjects. In SD, due to a taxonomic processing disorder, thematic processing is favored. Such an internal reorganization emphasizes that SD constitutes a good clinical model for the disorganization of semantic processing, and not a good model for the loss of the semantic system, in the sense of a unique system.