Fear can be rational or irrational. That said, there's a 'sense' in which it doesn't always matter whether one's fear is rational or not. That's because even if a particular fear happens to be irrational, that doesn't necessarily make it any less troubling. In fact, irrationality doesn't "negate" fear in the least-- and, in some cases, it can even breed terror.
Students often ask me about a saying from Jesus’ Sermon on the Mount: “… if your eye is good, your whole body will be full of light; but if your eye is bad, your whole body will be full of darkness” (Matthew 6:22-23). It’s an interesting saying to study, because it requires us to look carefully at the context, at the Old Testament background, and at some unusual Greek and Hebrew idioms. More importantly, once the passage is understood clearly, it illuminates a key kingdom principle.
Biola University