Jeremiah 10:8

But they are altogether brutish and foolish: the stock is a doctrine of vanities.

Those who put their trust in idols are altogether brutish and foolish, even while they profess themselves to be wise. The wooden stock itself is a vain teaching, a doctrine of vanities that instructs its followers in emptiness. To follow such an image is therefore to embrace folly under the claim of wisdom.

The verse warns against accepting falsehood in place of the living God. That living God is the Creator, in contrast to the lifeless work that can teach nothing true. To turn from Him to the stock is to exchange reality for vanity.