Vygotsky understands language and thought as modes of accessing reality. They differ from perception with respect to their presenting reality in a generalized manner; perception, instead, presents reality in its particularities. Vygotsky challenges the basic, underlying assumptions about the nature of language and thought, held by virtually all psychologists of his day. His critiques are not only rigorous and compelling. Even more, his positive proposals, in place of these old assumptions, are stunning and have born out to be more truthful. Developmental psychologists have taken on his insights and proven them to be explanatorily powerful.
Here's a summary of some of his main ideas. Previous thinkers either assumed that language and thought totally independent capacities, having no influence on one another. Or, they assumed that the two are merely identical with one another. Vygotsky's thesis opposes both extreme assumptions: instead, one's capacities to speak and to think are interdependent, each shaping the possible expressions of the other capacity.
Piaget, the most influential of Vygotsky's predecessors, was spot-on in aiming to understand the development of linguistic thought by examining its ontogenetic stages. But his main failing is his view that human infants start off as solipsistic individuals, and so language use progresses in the direction from egocentric-driven usages to more socially-driven and adapted usages. Piaget believed that infants are capable of only "autistic" thinking, which in his time meant thought that is not adapted to reality; that is, it is hallucinatory and illusory thought that is not responsive to real environmental conditions and that does not adapt in accordance to feedback given by the environment. (This is largely influenced by Freud).
Against this, Vygtosky argues that such "autistic" or imaginative thinking is a relatively late development. Infants are born in attunement with their environments; it is evolutionarily critical to be so, to be constantly responsive and sensitive to real events. Only later can children learn to recombine concepts learned from real experience in imaginative ways, and language is critical for enabling this activity. Moreover, language is essentially communicative from the start, rather than private and egocentric. We first learn language in dialogue with other people, and only later on discover ways to use speech when they are not physically present. For example, Piaget identified "egocentric" speech (the expression of autistic thinking) is not actually solipsistic. Vygotsky's empirical work showed that children talk to themselves precisely when they face challenges and need guidance. They use words to tell themselves instructions and engage in "dialogue" with themselves. This is with the aim to enable greater focus on the tasks that are needed to overcome challenges. Older children are able to "internalize" this speech, or to think linguistic thoughts, rather than utter them overtly.
The development of one's conceptual repertoire is closely tied to the development of speech. The meaning of words are conceptual in nature. Vygotsky examines the development of concepts in chapters 5 and 6. Vygotsky distinguishes between "scientific" and "spontaneous" concepts. The former are concepts that must be explicitly taught to us, in order to be learned, whereas the latter are concepts that we naturally form as we experience the world. Each kind of concept acquisition enhances and enables the developmental trajectory of the other. When we learn scientific concepts, these provide sophisticated structures (e.g., the ways concepts of this kind can formally relate to each other, such as standing in a hierarchy of generality to particularity) which we can apply to our spontaneous concepts, thereby ordering and renewing them. Scientific concepts can only be learned on the basis of our current conceptual repertoire, which for children largely consists in spontaneous concepts. The famous term in developmental psychology "scaffolding" is based in these accounts.
The core of Vygotsky's theory, found in the last chapter 7, is that the developmental tracks that mark the progression of thought and language are distinguishable but deeply interrelated. Overt speech advances from parts to whole (i.e., a few words to sophisticated sentences), while thought advances from whole to parts (i.e., a nebulous overall thought to nuanced, articulated thoughts).
Language and speech move in "opposite directions" in this sense, but are unified. When a thought is immature and nebulous, it can be expressed in a single world. As a thought becomes more differentiated, it can be expressed in more sophisticated sentences. Overt speech can serve as a tool that enables one to create greater nuances in one's thoughts, and with more nuanced thought, one is capable of more sophisticated speech. Thought must undergo ontological changes in order to become mature speech, and so thought and speech are not identical versions of one another, simply laying in different media, as some previous thinkers assumed.
Vygotsky also explores how young children don't distinguish between the referent and the meaning of words at the beginning of language development. That is, a child, in uttering a word and apprehending its meaning, conflates this meaning with the real object in the world to which the word refers. I am fascinated by this finding; I wonder whether this infantile tendency is still latent in adult behavior, where most of us find us compelled at times to make prayers or wishes. Such practices presuppose that the meaning that unfolds in language use can causally interact with objects in the world itself, if not even being identical to those objects. When we engage in wishful thinking, it is as if the desired, new arrangements of objects that we configure in our language now are real, manifested in the world itself.
Vygotsky has fascinating insights about the process by which the capacities for overt, verbalized speech can be "internalized" or transformed for the generation of inner speech, or linguistic thoughts. He starts of with the principle that the more familiar with one another two conversational partners are -- and the greater the shared background knowledge or context is between them -- the more "abbreviated" their speech can be. Between two best friends, one can just utter "Yeah..." with a particular intonation, and the net semantic meaning of this utterance can be equivalent to that of many sophisticated sentences.
Vygotsky points out that in engaging in inner speech, we essentially speak to ourselves. When we relate to ourselves, the dynamic this affords is loosely similar to that which emerges between two conversational partners (Vygotsky does not state this explicitly, this is my interpretation on what his premises must be). We know ourselves quite well, or we literally are ourselves. This makes for the highest degree of shared background knowledge and context between the "conversational partners" that are our self-conscious and our implicit selves. So this enables the most extreme extent of abbreviation in speech. When we think linguistic thoughts, we don't have to use much explicit wording, in order to express very complicated thoughts, which could take long discourses to express if we were speaking to a literal, other person.
Vygotsky also explores a point that is shared with late Wittgenstein. He points out that the meaning of a word can consist in meanings originally based in words usually associated with that word, or in the overall discourse or context in which that word appears. For example, the meaning of "The Brothers Karamozov" can consist in the vast stretches of meanings that make up this epic novel as a whole. We can apprehend all of this vast meaning in a nebulous way at 'one fell swoop'.
Overall, this book is rich is insights and ideas about basic processes in psychological development. It is a fascinating read. My only complaint is that many ideas presented in the body chapters of the book do not seem connected to the core of Vygotsky's theory, presented only in the last chapter. So the book feels somewhat disjointed. The heart of Vygotsky's critique of previous thinkers is found in chapter 1. Chapters 2-3 involve elaboration on Vygotsky's critique of his major predecessors. Chapters 4-6 involve Vygotsky's argument for the importance of a genetic/developmental account of language, and findings from his empirical research -- this culminates into the principles of his theory, which is laid out in chapter 7. I'd say the first and last chapters are the most interesting and important, and the rest of the book could be skimmed.
I'd recommend this book to anyone interested in naturalistic approaches to understanding semantic meaning or language.