It is said that Newton's laws of motion or laws of Quantum Mechanics posses time reversal symmetry but the second law of Thermodynamics does not. What I understand by the first part of the sentence is the following. The dynamical state of a system changes with the increase of time. The state at time 't+dt' is different than that at time 't'. If somehow time goes in the reverse direction from 't+dt' to 't' we will get back the state what we originally got. Please help me know whether this is a correct thinking.