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I'm learning about GPs, and one thing I don't quite understand is how the posterior works. Consider this figure:

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Rasmussen and Williams say:

Graphically in Figure 2.2 you may think of generating functions from the prior, and rejecting the ones that disagree with the observations... Fortunately, in probabilistic terms this operation is extremely simple, corresponding to conditioning the joint Gaussian prior distribution on the observations.

To formalize a bit, given this joint distribution,

$$ \begin{bmatrix} \mathbf{f}_* \\ \mathbf{f} \end{bmatrix} \sim \mathcal{N} \Bigg( \begin{bmatrix} \mathbf{0} \\ \mathbf{0} \end{bmatrix}, \begin{bmatrix} K(X_*, X_*) & K(X_*, X) \\ K(X, X_*) & K(X, X) \end{bmatrix} \Bigg) $$

the conditional distribution is

$$ \begin{align} \mathbf{f}_{*} \mid \mathbf{f} \sim \mathcal{N}(&K(X_*, X) K(X, X)^{-1} \mathbf{f},\\ &K(X_*, X_*) - K(X_*, X) K(X, X)^{-1} K(X, X_*)) \end{align} $$

What I don't understand is how samples from this conditional distribution always "agree" with the observations? Aren't the samples $\mathbf{f}_*$ still instances of Gaussian random variables?

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To be slightly more explicit about what I think your question is:

Yes, samples from the posterior are Gaussian everywhere, including exactly at the previously-observed points. But, in this "noise-free" setting, the variance at those points is 0 – so a Gaussian with variance 0 is always going to be exactly its mean.

It's easiest to see this in the case where we condition on only point, $X = X_*$, in which case the conditional variance becomes $$K(X, X) - K(X, X) K(X, X)^{-1} K(X, X) = 0,$$ and the conditional mean is $$K(X, X) K(X, X)^{-1} \mathbf{f} = \mathbf{f}.$$

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Yes, the samples $\mathbf{f}_*$ are once again samples from a multivariate gaussian distribution, but note now that the mean and covariance of this gaussian have changed.

Before we have observed any data, the distribution of $\mathbf{f}_*$ corresponding to input values $X_*$ are assumed to be drawn from the gaussian distribution $\text{N}(0, K(X_*, X_*))$. However, after conditioning on the observed data $\mathbf{f}, X$,note that the mean and covariance matrix of the corresponding conditional distribution $\mathbf{f}_*|\mathbf{f}$ have now changed to be $K(X_*, X)K(X, X)^{-1}f$ and $K(X_*, X_*) - K(X_*, X)K(X,X)^{-1}K(X, X_*))$ respectively. In this way, the changes to the posterior mean and covariance reflect the information gained by the observed data, and this posterior distribution will produce samples that align closely with the data.

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