this brings up the interesting topic of the intersubjectivity of the real world. we really do in significant ways "share" the world around us. It is public information, accessible by almost anyone who wishes to see things "out there". The interesting thing is that we can see substantially the same things, describe them in similiar terms and interact with them in ways that are repeatible and manipulatible. This is what science does on a systematic, longterm way that involves lots of very different people.
intersubjectivity, not objectivity, not subjectivity, emerges from this agreement with what we find out there. The commensurability of such intersubjectivity is really amazing given not just the problems philosophy has shown us are there (the matrix, brains in the vat, decartes demon etc) but the extraordinary depth of the system as well.
now, you can say that "aren't all opinions biased?" in such a way that it tends to lead to a radical solipisim or you can say it in such a way that it leads to science, that is all opinions may be biased but some are more biased towards being in conformity with the real world and some are more biased towards conforminty with private personal worlds that only that person has access to, and often can not really describe what they have found there when they return to us to tell us what they have found there.
science can, and does, and if you learn something of it, then the statement "aren't all opinions biased?" leads to agreement that conformity with reality is more truthful than is lack of conformity with it, or conformity with a private internal reality.