I came across the statement quoted above in Book 4 of Lucretius which is relevant to the way we discuss the role of reason in relation to the senses. At least in my own case I need to emphasize this point more during our discussions:
Nor in this [shadow illusions] do we admit that the eyes are in any way deceived. For their function is to see where light and shade are. But whether or not it is the same light, and whether the shadow that was here is the same one as is passing over there, or whether rather it happens in the way we said a moment ago, this falls to the mind's reason to discern. The eyes cannot discover the nature of things. So do not trump up this charge against the eyes for a fault which belongs to the mind.
(Long and Sedley The Hellenistic Philosophers)
I frequently comment on Lucretius' repeated use of the statement made first in 1:146 that: "This terror then, this darkness of the mind, must needs be scattered not by the rays of the sun and the gleaming shafts of day, but by the outer view and the inner law of nature; whose first rule shall take its start for us from this, that nothing is ever begotten of nothing by divine will."
I would presume that the point being made in both locations is approximately the same. it's not by observation alone, but by proper processing of observations through the mind, that we learn the true nature of things.