Our Gemara on Amud Beis discusses the implications of a person who appears calm but perhaps he is not. The Gemara sometimes rules that a verbal commitment is binding as there is a clear indication of serious intent, while other times we assume he was under some kind of duress and offered an empty verbal assurance but made no kinyan (physical act of transfer or acquisition) to solidify the transaction. The actual halachos are complex however we will focus on one scenario whereby the Gemara discusses a gentleman sipping his beer. The Gemara wonders if this is an indication of a sanguine attitude and his verbal commitments are not under financial duress therefore taken seriously, or quite the contrary, he is anxious and drinking beer to calm his nerves and is under duress.
Regardless of how the Gemara chooses to interpret this behavior, we see a sensitivity to the complexity of interpreting a person’s internal thoughts versus his external behaviors and actions. We saw earlier in our Psychology of the Daf blogpost for Bava Metzia 51 about the importance of noticing a person’s expressions and being more attuned. Additionally, there is a often confusion about what to believe in regard to reading another person’s thoughts. Everyone likes to say, “Don’t be a mind reader”, but emotions are clearly displayed on a person’s face. With intuition and moderate practice and sensitivity, a person can accurately read another person’s emotional state, even by noticing what are called micro-expressions. Micro-expressions are split-second unconscious leaking of emotions that the person may try to consciously conceal. Even though we can read emotions accurately we make mistakes in attributing a particular set of thoughts to the emotions. For example, a teacher or parent may accuse a child of lying because he appears nervous, might very well accurately perceive the emotion of anxiety but misattribute it to thoughts of deceit. The child might be telling the truth, just be and anxious that he won’t be believed.
In summation, human behavior is easy to observe and, to some extent, even subtle emotions can be accurately perceived. But indeed thoughts are impossible to read.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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