Language and thought: Piaget and Vygotsky

Piaget argued that representational thinking precedes language. A child must first be able to hold an object or idea in mind (a mental representation) before they can use a word for it.
For example, a baby develops object permanence (knowing an object still exists when hidden) before using words for objects.
So language depends on cognitive development: it reflects the thinking a child can already do. Young children's early speech is egocentric (focused on themselves) and becomes more social as their thinking develops.
Strengths: fits the order in which children develop; supported by observations of children's thinking before speech.
Weaknesses: it may underestimate how language helps thinking; for example, learning words can help children form new concepts.

Vygotsky argued that language and thought start separately and then join.
Pre-linguistic thought: thinking without language. A baby can recognise a parent or solve simple problems without words.
Pre-intellectual language: using sounds without thought behind them, such as babbling, crying or copying words to get attention.
At around age 2, the two come together: thought becomes verbal and speech becomes rational. Children talk aloud to guide themselves (private speech), which later becomes silent inner speech.
Language is learned through social interaction, and then shapes thinking.
Strengths: recognises the importance of social interaction; explains why children talk to themselves when solving problems.
Weaknesses: inner speech is hard to observe and test; the theory is less detailed about how the two join.

The key difference is which comes first, and whether language just reflects thinking or shapes it.