For as our Lord saith: The lanthorn (or light) of thy body is thy bodily eye. Right so may it be said, that the lanthorn of thy soul is reason, by the which thy soul may see all spiritual things. By this lanthorn mayest thou find Jesus, that is if thou hold up this lanthorn from underneath the bushel, as our Lord saith: No man lighteth a (candle or) lanthorn to set it under a bushel, but upon a candlestick. That is to say, thy reason must not be overlaid with earthly business, or vain thoughts, and earthly affections, but always upwards, above all vain thoughts and earthly things as much as thou canst. If thou do so, thou shalt see all the dust, all the filth and small motes in thy house (for He is light itself), that is to say, all fleshly loves and fears in thy soul. (John Climacus)

Reason will illuminate, if not obscured by clutter.

Reason is especially susceptible to being distracted by pride, lust, envy, and such.

But it is also with reason that we can see the clutter and choose to clear it away.

Reason combined with wholehearted purpose is very bright.

Reason joined to God's purpose is a great beacon.

The image is of the rose window at Freiburg Cathedral.