- James Brady
When training an artificial neural network, a simplified version1 of the classic workflow is:
- Set up an neural network with randomly-weighted neurons
- Feed an example from the training set into the network
- Calculate the difference between the actual and expected output
- Use backpropagation to update the weights of all the neurons in the network
- Go to #2 until you have processed the entire training set
What does this have in common with training a neural network?
To a single neuron, this is how training looks:
- Cue: accept inputs from neurons in the previous layer
- Routine: calculate the sum of the weighted inputs and apply the activation function
- Reward: during backpropagation, update our input weights according to the loss function
In fact, the analogy also works at the level of the network as a whole:
- Cue: transform our input example and input it into the first layer of the network
- Routine: the network processes the input through its layers to produce a result
- Reward: calculate how accurate the result is – compared to the labeling of the input example – and backpropagate
So, from a process perspective there do seem to broad similarities between how we – as humans – form habits, and how we perform supervised machine learning on neural networks.
Generalising habits: System 1 thinking
In Thinking, Fast and Slow, Daniel Kahneman draws a distinction between System 1 and System 2 thinking. The former is responsible for fast, cheap, lossy, inaccurate, and unconsious thought; the latter is responsible for analytical, reasoned, slow, expensive, and conscious thought.
Habits and habitual behaviour are archetypal examples of System 1 thinking: we can solve extraordinarily complex problems without even noticing it – for example driving a car while our mind wanders a little.
Do neural networks behave like our System 1 thinking?
Some hallmarks of System 1 thinking are:
- it's very quick
- it happens unconsciously
- it's well-suited to searching for patterns
- it requires training through repetition and reward
- it gives simple answers to complex questions
- it's prone to cognitive errors
The similarities between these attributes and the behaviour of trained neural networks are striking.
Admittedly, I cherry-picked these attributes to some degree but – apart from certain human-centric cognitive errors that we suffer from – it's hard to find a single characteristic of System 1 thinking which isn't also evident in artificial neural networks.
What does this mean for machine cognition?
Kahneman doesn't imply that his System 1/2 abstraction represents an actual underlying psychophysiological distinction: both systems are powered by the same underlying hardware – neurons – after all. On the other hand, fMRI experiments show that System 2 is associated with increased frontal and parietal cortex activity, so perhaps there really is a structural difference between the two systems.
Whether there is a physiological distinction between the systems or not, it seems clear that there is a wide spectrum along which cognition can happen – with System 1 at one end and System 2 at the other.
As described above, supervised machine learning looks to be at the same end of that spectrum as System 1: from both a process and behaviour perspective.
The two clearest conclusions from this are:
1. Artificial General Intelligence won't come from supervised machine learning
AGI is a dream or a nightmare depending on whom you talk to.
Either way, although supervised neural networks can take you deep into the uncanny valley, it seems like there will always be something missing if they just get better and better, faster and faster, at System 1-like responses.
2. We shouldn't expect neural networks to be able to explain their decisions
Yes, we could dump out the weights of all the neurons to fully "explain" a neural network. However, in the same way as performing that procedure on a human wouldn't "explain" their personality, we don't gain real insight from merely understanding the hardware.
There has been some progress on generating commentary alongside a particular decision but its domain is very limited. Many leading AI researchers have shifted in recent years to focussing on the statistical outcomes of the AI and optimising there – rather than demanding a wholesale account of each individual decision.
Neural networks just don't work in a logical, considered way which can be dissected and analysed afterwards. There is no rational answer to be explained so it would behove us to stop asking the question.
A more complete version of this would include careful selection of training, dev, and test sets, along with changes to the network architecture, hyperparameters, regularisation, … However, at its core, this simplified version is where the kernel of automated learning happens. ↩