The principle of Occam's Razor is the principle of conceptual minimalism - that "entities should not be multiplied except from necessity". This means that axioms and consequent beliefs, theories, suppositions, guesses, should be as simple as possible, but as complex as necessary, to accommodate the accepted data. If ideas are constructed other than as dictated by the data then there is no sufficient control over them; the salutary discipline of correspondence is lost and the seductiveness of runaway coherence is allowed free rein. Ideas degenerate into merely plausible fantasy perhaps needing the sustenance of further plausible fantasy. This rescue of fantasy by resort to further fantasy is termed rationalisation. "Oh what a tangled web we weave when first we practise to deceive" is no less true when the practised deception is tacit self-deception.
The irony of this principle being attributed to a fourteenth century cleric, when classic Christian theism was at its magnificent zenith, is that it does much to demolish latter-day religion, as ordinarily understood, while it tends to reinforce sceptical liberal secular humanism and to rescue such humanism from lapsing into yet another pseudo-religion. It has been, ideologically speaking, 'downhill all the way' for Christianity since the days of the putatively wise William. Secularists should take heed.
Wielding the famous razor, it is claimed that the minimum necessary secular liberal humanist assumptions (that can be wrong but which humanists gamble on being right) can be expressed as follows:
So, 1) Humanism is intellectual.
So, 2) Humanism is realist, materialist and empiricist.
Humanists see the need to resist the temptation to place a proposition higher up the scale of 'racing certainty' than is strictly necessary in the light of empirical data. We depend ultimately upon empirical data but blind faith in such data is itself a trap. 'It has always been so, so it will always be so' is the fallacy of the absolute dependability of induction (inferring general truths from particular cases). We even have to be wary of that. We have to act as though induction were dependable but, paradoxically, we have to accept that it is not absolutely dependable - induction, we recognise, is a necessary risk. (The parable of Russell's Chicken is noteworthy at this point - a certain chicken was fed daily by the same man at the same time and the bird assumed, on blind faith in the absolute dependability of experience, that this would go on for ever; one day the man came, as usual, and wrung its neck).
So, 3) Humanism is sceptical both in face of ideas and in face of data.
So, 4) Humanism is freethinking and open minded.
It is no accident that dualism presents the unsolved 'mind-body' problem. There is, in principle, no such problem; it is the price of dualism - not a first-order problem at all.
So, 5) Humanists are, philosophically, monists and materialists.
So, 6) Humanism is liberal, democratic and 'green'.
[This is not to say that the Liberal, Democratic, or Liberal-Democratic or Green Parties are necessarily humanist or that humanists are necessarily attracted to such parties in practice; parties and humanists are not always what they are cracked up to be!]
Most human beings subscribe to the basic moral values of truth, kindliness and respect for other people, for their personal autonomy. (Or, to be more precise, few people would admit to making virtues of the opposites of these values).
Since we enjoy no revelation and admit of no a priori certainties, our humanist ethics is not heavily prescriptive.. Ethical inquiry is the means whereby these values can be actualised in practice - including, if necessary, the means whereby these values can be reconciled with one another.
So, 7) Humanist ethics leans towards rule-utilitarianism rather than to absolute rules.
There are however basic differences between modern humanism and traditional religion (and in what follows the adjectives 'traditional' and 'modern' are to be taken as understood; there are many minority religious attitudes to which the following remarks do not necessarily apply).
Humanists properly regard their axioms as for ever questionable; mostly, those axioms cannot be proved without circularity. The senses cannot validate sense data - we have to assume that our senses give us dependable guidance as to a real 'out there'. Likewise, logic cannot validate logic - "it would wouldn't it."
Religious believers rest upon the perceived absolute truth of their axioms, their dogmas.
So, 8) the humanist's axioms are perceived as his Achilles' Heel; the religionist's dogmas are perceived as his Citadel of Truth. A corollary of this is that Humanists accumulate a body of revisable experience while religionists accumulate a deposit of faith.
9) deduction (arguing from the general to the particular) is a risk.
Humanists cannot 'do business' with those dogmatic religionists who pin their faith to absolutes - any more than the different kinds of absolute dogmatists can, honestly, do business with one another. But there are other religious believers who pin their faith in honest inquiry - even if humanists do not necessarily agree with the answers they suggest and see fit to believe. Humanists are not religious as ordinarily understood; humanism and religions (of both the dogmatic and the inquiring kinds) necessarily interact in respect of epistemology, ethics and the perennial question what does it actually mean to be human?
So 10) Humanists can and should maintain dialogue with the more questioning religious believers and to do so is to mutual advantage.