When a Semantic Dementia patient doesn't know what something means, has the patient lost the "building blocks" of semantic memory, or lost the ability to fit the blocks together? Our view of semantic memory is that it involves both knowledge content (blocks) and processes (building) that act on that content. Such processes include categorization. We taught complex, semantically meaningful novel categories by different categorization processes to three subgroups of FTD: Patients with semantic deficits (Semantic Dementia, or SD), patients with grammar deficits (Progressive Non-fluent Aphasia), and patients with executive and social impairment. We manipulated category content and categorization process during training such that categorization judgments at test would reflect varying degrees of executive resource limitations. Paradoxically, the SD subgroup was best able to categorize our novel stimuli. Our results suggest that SD patients have lost the "blocks" for familiar objects but retain the "building" process; when provided with blocks, they can fit them together. In contrast, the other patient groups seem to have trouble building, even when the blocks are provided. Hence, SD appears to partly reflect degraded semantic knowledge, while the other forms of FTD appear to partly reflect loss of executive function.