I think it was sometime last year that I had a Christian, in the course of a friendly but challenging conversation, pull the 'everything is subjective' argument on me. I gather the argument is basically "All of our experience is subjective to some extent, framed by our human minds, and none of our experience is absolutely certain, so I can believe whatever I want." It's an inventive defence; I shall not accuse it of being borrowed from postmodernists (atheist or otherwise), for postmodernists owe something to Soren Kierkegaard, and Kierkegaard was a devout Christian who did use this argument in a fashion at least vaguely similar, albeit with much more depth and the insertion of several other fascinating ideas that I don't quite agree with but which expand my mind nevertheless. At the time, I simply conceded that arguing against that idea might take a little more thought than being able to simply point to the evidence. But I've thought further on the subject, and by now I think I can offer a defence.
Let me start by conceding outright that of course we are trapped in our own minds; we do filter what we see through a subjective lens. Yet objectivity makes its way in, nevertheless. Suppose we're wandering along in our subjective experience, dum de dum, subjective subjective subjective, and we notice categories, yes? For example, some of the things we experience are experienced as being 'black' (whatever that means), some are 'white', some are 'coloured'. The edges of those categories might be blurred, but we can generally say that some things definitely fall more into one category than another. The boundaries between categories can also shift depending on time and place. Some things feel cold to the touch, some things feel hot, but which way such things are felt depends heavily on what we have been touching previously.
Now, as we're wandering along in our subjective experience, we might notice a category of perceptions which do not tend to shift depending on time and place and the experiencer and the method of experiencing. For example, temperature as measured by a column of mercury doesn't shift in the same way that subjective perception of heat does. To take a more interesting example, we can measure the structure of a crystal with X-ray diffraction, and we can measure it with an electron microscope, and when we do, we find a strong overlap between the two different methods of perception -- things which don't shift depending on the method of measurement. Or, yet another example, working from the hypothesis that all animals have a common ancestry, we might guess their relationships by looking at them, and we might guess their relationships from similar structures in their DNA. If the guesses tend to agree, well that's an indication that this 'distance of relationships' property really is measuring something that belongs somewhat to the category of perceptions which do not depend heavily on the method of perception.
So tell me, what name do we give to this category of things which remain constant depending on the method of perception? Why, 'objective', of course! The word 'objective', like most words, has some variations in meaning, but this is at least one definition which seems to me to be well thought out, and worth keeping. The observation that there is some subjectivity in all of our experience does not negate the usefulness of this category. Moreover, if we wish to know what things we can rely on to remain constant and predictable, it is in the category of objective perceptions that we should be looking.
Science is an attempt to identify members of this 'objective' category. Given this fact, I am inclined to consider the notion of consilience as being science's central point. Consilience refers precisely to the sort of situation I have described above -- to situations where several different types of measuring, of guessing, of perceiving all come to the same conclusion. If my characterisation of objectivity is allowed to stand, consilience is by definition objectivity's hallmark. As such, any idea which wishes to attain the status of 'objective' must pass a scientific test -- a test which varies the conditions and/or the method of perception, and still gets the result which our earlier generalisations would have predicted. The bigger the variation in circumstances of the test, the stronger an indication of objectivity it is.
Ideas that do not show this sort of consilience cannot be said to be 'objective' in this sense. People's ideas of God, for instance, depend heavily on the culture in which they were brought up. If there is any consilience to be found here, it will certainly not be found in the extraneous trappings and extra details which vary from place to place. Yet the variations are so large, encompassing even differing ideas about how many gods there are, that it is difficult to see how any objectivity in this matter may be found. Perhaps someday somebody will show an idea of God which does stand up to scientific tests, which does display consilience. Until then, though, I remain an atheist.