Lead is an independent module which will contain information about the person, their company and the opportunity that we have on them. Whenever the lead shows enough potential, it can be qualified. In Freshsales, a lead can be qualified by converting it to a contact (which contains personal information), account(which contains company information) and deal(which contains opportunity information).
Each of these three modules are dependent and the conversion marks a step forward in the sales process. This process cannot be reversed. Hence, a contact cannot be converted into a lead.