Most definitions of ontology, viewed as a "specification of a conceptualization", agree on the fact that if an ontology can take different forms, it necessarily includes a vocabulary of terms and some specification of their meaning in relation to the domain?s conceptualization. And as domain knowledge is mainly conveyed through scientific and technical texts, we can hope to extract some useful information from these texts for building ontology. Indeed, we use names for denoting concepts and linguistic relationships such as hyponymy can be viewed as a linguistic translation of the subsumption relationships between concepts. But is it as simple as this? In this article we shall see that the lexical structure, i.e. the network of words linked by linguistic relationships, does not necessarily match the domain conceptualization. In other words, the conceptual structure derived from the lexical structure - if we consider a word as a lexicalized concept - does not generally fit the conceptualization of the domain. We have to bear in mind that writing documents is the concern of textual linguistics, of which one of the principles is the incompleteness of text, whereas building ontology - viewed as task-independent knowledge - is concerned with conceptualization based on formal - and not natural - languages. We should also bear in mind that using rhetorical figures in writing text, like metonymy and synecdoche, modifies the perception of any concepts we may have. While we can extract some useful information from texts, ontology cannot be directly built from them.