(A) think about something
(B) try to find out if your thoughts match reality / try to test your thoughts
(A) observe something (or remember an observation)
(B) try to interpret / to understand the observed phenomenon
“Mental laziness” leads to a neglect of step (B). A mental lazy person doesn't think much about the phenomena he/she sees and does not care much if his/her thoughts are closely related to reality or not.