Necessity and sufficiency in subjective view For example, we have: A SSL module is needed for Apache to be secured A beautiful woman is needed to have a true love. In these two example, how can we determine exactly a condition is necessity and/or sufficiency? In the case of web server, an inexperience person may view it as necessity and sufficiency, while another very experienced person may view it as neither needed or sufficient to satisfy the condition of a secured web server. Same for the beautiful woman example. Some people view it that way as being necessity and sufficiency, while others may not need either one. So, how can necessity and sufficiency be correctly applied? The above examples are pretty subjective, so does it mean the necessary condition to apply is first and foremost, the situation must be objective?

Doxastic logic is a form of modal logic that deals with subjective beliefs. For instance, your first statement could be formalized as "secured (Apache) $\to\exists$ SSL module", and then the subjectivized version "Some people believe that an SSL module is needed for Apache to be secured" might be "$\exists p:B_p$(secured (Apache) $\to\exists$ SSL module), where $B_p$ is the operator for the beliefs of person $p$.