All attempts to sum up David Hume's views on religion stumble when they turn to what is, on the face of it, the most basic question of all: was he an atheist or an agnostic?

Ironically, contemporaries who described him as an atheist used to include his worst enemies, while those who are now keenest to apply the label claim him as a best friend. In both cases, however, the reasoning is the same: Hume is so critical of religion that his refusal to simply come out as an atheist must have been the result of a simple fear of the troubles such a professed disbelief would have caused him. "The great infidel" as James Boswell called him, stopped short of embracing atheism for purely pragmatic reasons.

The case against, however, is also fairly convincing. Hume was above all a sceptic who cautioned against human reasoning over-reaching itself. This cuts both ways – against people who claim to understand the nature of God and those who claim to be sure he doesn't exist. If you add to this some of Hume's recorded comments, such as his remark to Baron d'Holbach that he had never even met an atheist, and the case for his agnosticism seems closed.

That the right answer is still not crystal clear is not due to any confusion on Hume's part, but to ambiguities in the meanings of atheist and agnostic. Consider the case of Bertrand Russell, another atheists' hero who preferred to call himself an agnostic. In his essay Am I An Atheist Or An Agnostic? he wrote:

As a philosopher, if I were speaking to a purely philosophic audience I should say that I ought to describe myself as an agnostic, because I do not think that there is a conclusive argument by which one prove that there is not a God. On the other hand, if I am to convey the right impression to the ordinary man in the street I think I ought to say that I am an atheist, because when I say that I cannot prove that there is not a God, I ought to add equally that I cannot prove that there are not the Homeric gods.

There are two senses of agnosticism at work here. One is a suspension of judgment which leaves the mind of the doubter as open to belief as to disbelief. This kind of agnostic is genuinely uncertain as to whether God exists or not, and is unable to say which possibility is significantly more likely. Neither Hume nor Russell were agnostics in this sense. For them, God was not something whose existence they continued to seriously entertain. However, they were agnostics in another sense: unable to conclusively settle the matter, God becomes a kind of irrelevant distant possibility, albeit one they would not say definitely did not exist.

If there are two forms of agnosticism, then there are also two kinds of atheism. One is the definite belief that God certainly does not exist. In this sense, neither Hume nor Russell were atheists. But as Russell pointed out, there is another sense of atheist, in which belief in God plays no role and is not entertained as a credible hypothesis. In practice, this is life without belief in God, and is certainly very different from the kind of deeply uncertain agnosticism which holds belief and non-belief as almost equal possibilities.

For what it's worth, I think that it is more accurate to describe Hume and Russell as non-dogmatic atheists than agnostics, because what matters is not whether an atheist is 100% convinced she is right, but whether or not she is content to understand life, meaning, value and purpose in godless terms. To describe the non-dogmatic atheist as an agnostic makes the question of God's existence sound more open to her than it really is.

William James, in his essay The Will to Believe, goes some way to explaining why it is in some ways more honest and accurate for a supposed agnostic to prefer the atheist label. He argues that agnosticism is not a live option, because the choice between belief and non-belief is in a real sense forced:

We cannot escape the issue by remaining skeptical and waiting for more light, because, although we do avoid error in that way if religion be untrue, we lose the good, if it be true, just as certainly as if we positively chose to disbelieve.

He continues later:

To preach skepticism to us as a duty until 'sufficient evidence' for religion be found, is tantamount therefore to telling us, when in presence of the religious hypothesis, that to yield to our fear of its being error is wiser and better than to yield to our hope that it may be true.

What James is saying is that to live life without active belief in religion is, in effect, to live godlessly, as an atheist. I'm not sure that the choice is quite as forced as James maintains, but anyone whose absence of belief in God leaves them living their life as though he doesn't exist is living the life of an atheist. Hume was one such man. Whether he thought it justifiable to assert "God does not exist" or not, he was as godless a man as can be imagined. If that's not what he meant by atheist, then it's certainly not what most people mean by agnostic either.

Next week the moral philosopher Mary Midgley will start a series of blogs on Thomas Hobbes. For Julian's previous blogs on David Hume and religion, visit the How to believe series page