C. S. Lewis wrote:
The heart never takes the place of the head: but it can, and should, obey it.
According to this, should we follow the heart or head?
Perhaps reading the context of the quote will help make more sense of it. It is from The Abolition of Man:
No emotion is, in itself, a judgement; in that sense all emotions and sentiments are alogical. But they can be reasonable or unreasonable as they conform to Reason or fail to conform. The heart never takes the place of the head; but it can, and should, obey it. (source)
Lewis here is discussing the difference between reason and emotions, and pointing out that in some cases, our emotions are out of line with reason – we may recognize that we ought to like something, but we don't feel any liking for it (or vice versa). Lewis says that such discrepancies are defects in ourselves, like color blindness or tone deafness.
Thus this quote is saying that though the heart does not replace the head, it (the heart) can, and should, obey the head; or, put another way, we should seek to conform our emotions to reason.