How does reasoning work in large language models (LLMs)?

Large language models (LLMs) like GPT-4 or Llama perform reasoning by leveraging patterns in their training data and statistical relationships between words, rather than explicit logical deduction. When given a prompt, the model predicts the next token (word or subword) in a sequence by analyzing the context provided. This prediction is based on weights learned during training, which capture how frequently certain concepts or phrases appear together. For example, if asked, “What is 3 + 5?” the model doesn’t calculate the sum but retrieves the answer “8” because it has seen similar math problems and solutions in its training data. This process is powered by the transformer architecture, which uses attention mechanisms to weigh the relevance of different parts of the input when generating each token.

Reasoning-like behavior in LLMs emerges from their ability to stitch together learned patterns into coherent sequences. For instance, when solving a multi-step problem like “Alice has 5 apples; Bob gives her 3 more. How many does she have?” the model might break it down step by step: first adding 5 and 3, then stating the result. This mimics reasoning not because the model understands arithmetic, but because it has seen countless examples of such problems being solved incrementally in its training data. The attention mechanism helps maintain consistency by focusing on relevant tokens (e.g., “5,” “3,” “apples”) across the sequence. Developers can enhance this behavior using techniques like chain-of-thought prompting, where the model is explicitly instructed to generate intermediate steps (e.g., “Step 1: Calculate 5 + 3…”), improving accuracy by mirroring structured problem-solving examples from training.

However, LLMs lack true reasoning and often fail when faced with novel scenarios. For example, if asked, “If a bathtub holds 150 liters and drains at 10 liters per minute, how long until it’s empty?” the model might answer “15 minutes” correctly if similar examples are in its training data. But if the problem involves unusual constraints (e.g., “the drain slows by 1 liter/minute every 5 minutes”), it might struggle because the required step-by-step logic isn’t reflected in its training patterns. Developers can mitigate this by fine-tuning models on domain-specific data or designing prompts that guide the model to decompose problems. Ultimately, LLM “reasoning” is a statistical approximation of human logic, dependent on data quality, architecture, and prompting strategies rather than innate understanding.