Every thing, beings and unknown words when we first sense (see, hear, feel or smell) it, we have impression on our mind judging what could it be, what it does it do or have, and if it is good or bad. And according to may this first impression lasts longer.
During the final exams of geography in my sixth grade, an invigilator came towards me and picked up my copy suddenly. "Hey, your handwriting is great", she said. Then one of my friend popped out, "Like you said, he's also great at Physics too." "I can understand. Teachers know about it on the first day."
Like in this teacher's case, first impression sensed him a quality of me, which was true. We all decide what to eat just be seeing the picture in the menu regardless of what they used to make it or how others review it. We all choose many of the products such as booking a hotel, shopping online or buying a dress. First impression matters here.
In contrast to the positive side, many critics states, "Don't judge a book by its cover." Its because the look hardly have a direct connection between the content of the book. Another example, a senior student first thought I studied in third grade when I actually was in sixth. He was astonished when I faced him team in annual sports match. But he mocked me due to my height, and told that I don't deserve playing football with them. However, he appreciated my performance after the match.
So there is where the first impression don't matter. Impression comes naturally, nothing to be blamed. But its a question when it is applicable and when it is not. It is also fine to think and judge about that impression. But people had only worked to prove that it is the behaviour only that tells how it is. I agree with the statement to a same extent where it is always applicable.