The arithmetic sequences have a constant difference between two terms. This sequence does not match this criterion, look: 12-8 = 4; 8-5=3; 5-3=2. Then it is not arithmetic. Let's examine whether it is geometric. Geometric sequences have a constant ration between adjacent terms: given that 3/2 is different to 5/3, and both are different to 8/5 and so on, this sequence does not either match this requirement. Then the sequence is not arithmetic or geometric.