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Foundations

Deep Learning

Machine learning with many-layered neural networks — the technique behind modern AI.

Definition

Machine learning using neural networks with many layers. Each layer learns progressively more abstract features, which is why a deep network can go from raw pixels to "that is a cat" without anyone specifying edges or whiskers.

The "deep" is about depth of composition, not depth of thought. What changed in the 2010s was scale: bigger networks, more data, and GPUs to train them.

Depth is what buys abstraction without hand-designed features. In older vision systems somebody wrote an edge detector; in a deep network the early layers effectively discover one because edges are useful for the layers above. That is the whole argument for stacking layers, and it is why "more data" became a strategy.

How people use it
Sizing up a build.
"Speech recognition is a deep learning problem — you are not going to hand-write that."
Worked example

Why nobody writes feature extractors any more

Predicting churn from raw support tickets, years before LLMs.

The old pipeline had a hand-built feature stage — counts, sentiment words, escalation flags. Fine-tuning a model on the raw text beats it, because the model finds features nobody thought to enumerate. It also means you cannot read the feature list to explain a decision.

Sources
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