littlebabyjesus
one of Maxwell's demons
Language most certainly permits much greater complexity in thought. It permits much more creativity in thought too. If you think of thought in a non-linguistic animal as essentially a function of memory, shifting different concepts around to make better sense of your past, language adds a whole new set of internal possibilities. Now, with these tools in your memory and so available to your thought, the range of things you can think about is endless, things that you yourself have not experienced. Thought is a search for meaning – an improvement on the story of your life contained in your memory – but we can extend that search far beyond ourselves with language.Language: An external resource (set of public rules) that permits much greater complexity in thought. In particular, it gives us the means to think about thinking (reflexivity). That we identify ourselves as subjects is only possible due to language.
You have to be careful not to overstate that, though – I have thought of concepts for which I could not find a satisfactory word: concepts can exist without language, and complex ones at that. Also there is the possibility that non-linguistic animals think in pictures, and manipulate those pictures in a manner at least analogous to language.
I do not agree that we identify ourselves as subjects because of language. We construct a model within us – the contents of our consciousness – and use that model to look at ourselves in the world. Many other animals do the same, and fill their constructions of themselves in the world with meanings, as we do. That, combined with feedback, particularly from memory, provides the sense of self, that strange feeling of duality. Non-linguistic animals may have very different – and compared to us, impoverished – senses of self, but I do not doubt that it is right to talk of many animals as possessing some measure of subjectivity. Some more than others, of course: elephants are probably the likeliest of all, imo.
I'm not sure what you mean by language being an external resource. We develop our linguistic skills within ourselves. We require stimulus from the environment, of course. We need to have language spoken to us. But language acquisition is far from passive – we generate grammars ourselves, tweaking the rules of our grammars where they don't quite match the 'external' grammar. It is collaborative, I suppose, but then language developed to allow us to communicate with each other better – I'm guessing that was its primary selective advantage, and that improved thought was an 'accidental' by-product. If Gould is to be believed, such evolution is common – traits appearing as by-products of other developments, and only later finding any adaptive advantage (or never – if our nakedness is a by-product of the neoteny that has given us our ability for lifelong learning, for instance).
As an aside, and I know this isn't where you're coming from...
It is dangerous to draw conclusions from the impoverished nature of humans who are not taught language as children. We have developed to develop language, so it is no surprise that we cannot function at all well without it. If a child was not taught language, there will also be loads of other cognitive deficiencies due to their neglect. To compare that with, let us say, an elephant that has grown up in a secure family group, and learned everything that a baby elephant should learn, in its restricted way that elephant may have wisdom about certain things – even to us – may have worked out a particular way to use its trunk, for instance, or may just know exactly the right thing to do to comfort its own baby; it may well know that it will one day die, and perhaps faces that fact with equanimity. I have always found arguments for the crucial role of language in self-awareness unconvincing – it is very hard for us to imagine what it is like to be an elephant, but we shouldn't dismiss it as qualitatively inferior to what it is like to be us too quickly.
Damasio's model of a 'core' and 'extended' consciousness is useful here. We share the core consciousness with many animals – the model of 'now' that we experience. With our working memory able to place this now in a past and future, we have an extended consciousness that gives us a strong sense of self. I think at least some other animals also display evidence of an extended consciousness. As far as I've been able to find out, not enough work has been done on the anatomy of non-human brains to work out how their extended consciousness may work – ours works primarily with the prefrontal cortex providing space for a working memory, but I'd reserve judgement about whether this is the only way evolution has found to do it.