# 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?

-
You may want to read this thread and you should also make the title more informative. – Asaf Karagila Feb 23 '12 at 8:44
In both cases I would read needed as meaning necessary but not necessarily sufficient. – Henry Feb 23 '12 at 8:46
But is it really necessary also? Maybe a woman does not need to be beautiful to have true love. So, I think, to apply necessity and sufficiency in deduction, each of the conditions in a (P => Q) must be absolutely true, isn't it? – Amumu Feb 23 '12 at 9:04
It's just an example (as it is used in many other examples in this topic), to make thing simpler to read and comprehend without using Math notations. If you're about to encode such information for computer to deduce, how to make it not ambiguous? You can read more here: lisperati.com/tellstuff/reasoning.html – Amumu Feb 23 '12 at 9:38

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 $p$.