In the answers to this question it was shown, that a few licenses exist, that are only open source but not free software or the other way round. Still, most licenses are both or neither, so the overlap is very big.But the communities around open source are distinctive and both strong, and some members of one community even disagree with the other. That seems strange for communities that are so similar in definition and goals. It points to a subtle philosophical difference the two.So my question is: do such differences exist? And if yes: What are the differences in philosophy?
What are the philosophical differences between open source and free/libre software (if they exist at all)?
philosophy
I recommend visiting: Free software movement and Open-source movement for historical point of viewfree software for freedom, full article open-source-misses-the-point and When Free Software Isn't (Practically) Superior for a Philosophic point of viewPhilosophy of the GNU project is closely relevant with Free software movement and hence free software philosophy.Notable points (from GNU):-In addition to / Including @Michael Schumacher's answer.Open source is a development methodology; free software is a social movement. For the free software movement, free software is an ethical imperative, essential respect for the users' freedom. By contrast, the philosophy of open source considers issues in terms of how to make software betterin a practical sense only. It says that nonfree software is an inferior solution to the practical problem at hand.......  For the free software movement, however, nonfree software is a social problem, and the solution is to stop using it and move to free software.Practical difference:Open source doesn't take Tivoization as issue while for Free Software, it is ethical/social problem: (Quoted form Why Open Source misses the point of Free Software)...Even if the executable is made from free source code, the users cannot run modified versions of it, so the executable is nonfree.The criteria for open source do not recognize this issue; they are concerned solely with the licensing of the source code. Thus, these unmodifiable executables, when made from source code such as Linux that is open source and free, are open source but not free.Different Values:The idea of open source is that allowing users to change and redistribute the software will make it more powerful and reliable. But this is not guaranteed. Developers of proprietary software are not necessarily incompetent. Sometimes they produce a program that is powerful and reliable, even though it does not respect the users' freedom. Free software activists and open source enthusiasts will react very differently to that.A pure open source enthusiast, one that is not at all influenced by the ideals of free software, will say, I am surprised you were able to make the program work so well without using our development model, but you did. How can I get a copy? This attitude will reward schemes that take away our freedom, leading to its loss.The free software activist will say, Your program is very attractive, but I value my freedom more. So I reject your program. I will get my work done some other way, and support a project to develop a free replacement. If we value our freedom, we can act to maintain and defend it.Notable para from fear of freedom:The rhetoric of open source has convinced many businesses and individuals to use, and even develop, free software, which has extended our communitybut only at the superficial, practical level. The philosophy of open source, with its purely practical values, impedes understanding of the deeper ideas of free software; it brings many people into our community, but does not teach them to defend it. That is good, as far as it goes, but it is not enough to make freedom secure. Attracting users to free software takes them just part of the way to becoming defenders of their own freedom.Most GNU/Linux users were introduced to the system through open source discussion, which doesn't say that freedom is a goal. From Richard Stallman's TEDx video:Notable points (from OSI):-The mission of opensource says:Open source is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in.Quoted from opensource history:The conferees also believed that it would be useful to have a single label that identified this approach and distinguished it from the philosophically- and politically-focused label free software. Quoted from OSI FAQ:-The term open source was coined in 1998 by a group of people  the founders of the Open Source Initiative (OSI)  who also supported the development and distribution of free software, but who disagreed with the FSF about how to promote it, and who felt that software freedom was primarily a practical matter rather than an ideological one (see for example the entry How is 'open source' related to `free software'? from the OSI's original 1998 FAQ page).Following the coining of the term open source, some of those who adopted it did so because they too had philosophical differences with the FSF about the reasons why to promote such software, while others who adopted the term did so because of differences of opinion with the FSF about tactically how to support such software, even while sharing an ideological motivation. These two groups can and do overlap, of course, and some people use both terms, choosing according to context and audience.Also categories of free and non-free software from this answer which graphically explains difference/overlap i.e how opensource is free (mostly) and non-free (rarely).