'Idealism' is a term of wide latitude. I take it provisionally as the view that all that exist are (a) ideas in (b) a subject's mind. Generally a plurality of minds is assumed. If any philosophy matches this description it is Berkeley's Idealism.
▻ BERKELEY'S IDEALISM AS A TEST CASE
Then consider Berkelian Idealism. For Berkeley all that exist are minds and their ideas. Minds are entities, just not material entities - they are immaterial substances. They exist but are purely thinking or perceiving things. What they think or perceive are ideas. There is not just a single, conscious, immaterial substance : there is a plurality of such substances, namely all the separate immaterial minds.
Immaterial substances do not co-exist with material substances. For Berkeley there is no such thing as material substance. What we regard as material or physical entities or objects are nothing but collections of ideas. It's weird, probably wrong, but it's what Berkeley believed and what he argued for with considerable ingenuity.
▻ MINDS AND IDEAS : HOW RELATED ?
Berkeley is clear that minds and ideas are distinct. Neither is reducible to the other. Minds perceive ideas; ideas are perceived. Take Principles, §2 :
- MIND--SPIRIT--SOUL.--But, besides all that endless variety of ideas or objects of knowledge, there is likewise something which knows or perceives them, and exercises divers operations, as willing, imagining, remembering, about them. This perceiving, active being is what I call MIND, SPIRIT, SOUL, or MYSELF. By which words I do not denote any one of my ideas, but a thing entirely distinct from them, WHEREIN THEY EXIST, or, which is the same thing, whereby they are perceived...
▻ IDEALISM AND ACTIVITY
Minds, or mental substances, are perfectly capable of activity. Berkeley spells this out in Principles, §28 :
- I find I can arouse ideas in my mind at will, and vary and shift the mental scene whenever I want to. I need only to will, and straight away this or that idea arises in my mind; and by willing again I can obliterate it and bring on another.It is because the mind makes and unmakes ideas in this way that it can properly be called active. It certainly is active; we know this from experience. But anyone who talks of ‘unthinking agents’ or of ‘arousing ideas without the use of volition’ is merely letting himself be led astray by words.
▻ REALITY IS NOT COMPOSED MERELY OF IDEAS
If Berkeley held that reality consisted purely of ideas, he would be in trouble. How could ideas exist independently of minds that have them ? (This rules out the account of Idealism as the view that all that exist are ideas and that ideas exist by themselves.) But he holds no such view. Look at the quote from para 28 : 'I find I can arouse ideas in my mind at will'. In my mind - how could he arouse ideas in his mind if there were no minds but only ideas ?
▻ NO ENDORSEMENT OF BERKELEIAN IDEALISM
I am not trying to sell Berkeleian Idealism to you but to explain how and why your source, which aims to refute Idealism by 'showing' that Idealism cannot accommodate activity because it does not recognise entities that can be active, is mistaken.
Berkeleian Idealism runs into endless difficulties but this is not one of them. It's possible to argue, of course, that Berkeleian Idealism cannot really accommodate activity because there are no such things as immaterial substances to be active. But this is circular reasoning : it assumes that Berkeleian Idealism is false in order to refute Berkeleian Idealism. It has first to be proved that there are no immaterial substances before their non-existence can be used against Berkeley to show that he cannot accommodate activity.
G. Berkeley, 'A Treatise Concerning the Principles of Human Knowledge', 1710 : permanently in print and available online.