I would argue that the second definition is the most important, but that the first compliments it while the third loses the usefulness of the distinction.
Terry Eagleton gives no less than Napoleon pride of place in coining the word. But the idea behind it is straight-forward enough in European philosophy - that opinion or dogma, is an inferior kind thinking to more reasoned or scientific thought.
What Marx adds to that is that in the sphere of social thinking, spontaneous thought will tend merely to reproduce the status quo, while scientific materialism will penetrate beneath the appearances of the market to uncover the transient nature of capitalist society and the formation of the new society within the womb of the old.
In Marx's version mere ideology (or what is sometimes called 'false consciousness', though that it itself a flawed concept) is the natural product of the bourgeois standpoint. For the revolutionary proletariat, by contrast, only science will do. With nothing to lose but their chains, the workers have the capacity to look beyond the given state of things, that the bourgeois, condemned to insist that his is the only possible way, lacks.
Hence, for Marx, the working class is the 'universal class' that Hegel tried to find in the state officials, because only its immediate needs coincide with the transcendence of the status quo.
Marx also insists that illusion can only be dispelled with the dissolution of the state of affairs that demands illusions to sustain it (as PA Van Heusden quotes on the bottom of his posts). With that twist in the argument, Marx gives the philosophical idea of higher reason a materialistic twist. In the section on commodity fetishism in Capital, vol 1 Marx shows how the illusions of the market are 'real illusions', ie they correspond to real (though partial) existences of the market.
The vulgarisation of Marx by such as Manneheim and Simmel effectively reduces Marx's materialistic argument to a relativistic epistemology by rejecting the concept of the universal class. With that simple stroke, all knowledge is identified with interests, and we are immediately thrown back 2000 years to the wilful stupidity of the Sophists, like Thrasymachus. And this is what is called progress by our friends the deconstructionists.
(Incidentally, Charles, I am sorry I did not reply to your earlier post on this, which I inadvertently deleted. My only thought on that would be Lukacs' argument that the most important conflict in philosophy is less idealism v materialism than rationalism v irrationalism.)
In message <s7de6163.057 at mail.ci.detroit.mi.us>, Charles Brown
<CharlesB at CNCL.ci.detroit.mi.us> writes
>In re a definition of ideology (vs science) from a few days ago, Raymond
>Williams in _Marxism and Literature_, Chapter 4 ,"Ideology" says:
>
>"The concept of "ideology" did not originate in Marxism and is still in no way
>confined to it. Yet it is evidently an important concept in almost all Marxist
>thinking about culture., and especially about literature and ideas. The
>difficulty then is that we have to distinguish three common versions of the
>concept , which are all common in Marxist writing. These are broadly:
>
>(i) a system of beliefs characteristic of a particular class or group;
>
>(ii) a system of illusory beliefs - false ideas or false consciousness -
>which can be contrasted with true or scientific knowledge:
>
>(iii) the general process of the production of meanings and ideas.
>
>
>In one variant of Marxism, sense (i) and (ii) can be effectively combined. In a
>class society, all beliefs are founded on class position, and the systems of
>belief of all classes - or, quite commonly, of all classes preceding, and other
>than, the proletariat, whose formation is the project of the abolition of class
>society - are then in part or wholly false (illusory). The specific problems in
>this powrful general proposition have led to intense controversy within Marxist
>thought. It is not unusual to find some form of the proposition alongside uses
>of the simple sense (i), as in the characterization, for example by Lenin of
>'socialist ideology'. Another way of broadly retainng but distinguishing senses
>(i) and (ii) is to use sense (i) for systems of belief founded on class position
>, including that of the proletariat within class society, and sense (ii) for
>contrast with (in a broad sense) scientific knowledge of all kinds, which is
>based on reality rather than illusions..."
>
>
>
>CB
>
-- Jim heartfield