You've probably been warned more than once in your life not to make assumptions. This advice is far from accurate; it's necessary to make assumptions. You can't come to a conclusion about anything without making assumptions.
An assumption is a thought you have and presume to be correct. Based on that, you can come to a conclusion. The big difference between automatic versus critical thinking is:
The advice about assumptions should be, “Don't make assumptions without knowing how you arrived there or make assumptions you cannot validate.”
Have you ever left for work earlier than usual because the weather was bad and because you had to be there by a certain time? You assumed it would take longer to get to work because it was raining, snowing, or hailing. Why did you assume that? Because you've dealt with the situation before, and just about every time the weather is bad, the commute is slower. Therefore, your assumption is probably valid.
Let's say you're working on a project, and one team member you've never met is late to a status meeting. There is another status meeting one week later. Do you assume this member will be late? Because you've had only one experience with him or her, making that assumption would be a poor choice. However, if he or she were late to four out of the last five status ...