Sentence:  <font face="courier">The first-order procedure SP differs from the proposi-/  tional procedure CP&deg;<sub>1</sub> in an essential feature. Namely, CP&deg;<sub>1</sub>/  always terminates while SP may run forever as we have seen with/  the example immediately after (3.7). This is not a specific/  defect of SP. Rather it is known that first-order logic is an/  <WSD>undecidable</WSD> theory while propositional logic is a <WSD>decidable</WSD>/  theory. This means that for the latter there are decision pro-/  cedures which for any formula decide whether it is valid or/  not — and CP&deg;<sub>1</sub> in fact is such a decision procedure — while/  for the former such decision procedures do not exist in princi-/  ple. Thus SP, according to these results for which the reader/  is referred to any logic texts such as [End], [DrG] or [Lew],/  is of the kind which we may expect, it is a semi-decision/  procedure which confirms if a formula is valid but may run/  forever for invalid formulas. Therefore, termination by running/  out of time or space after any finite number of steps will/  leave the question for the validity of a formula unsettled. [...]</font> 
Meaning of the word marked with <WSD> tags:  Incapable of being algorithmically decided in finite time. For example, a set of strings is undecidable if it is impossible to program a computer (even one with infinite memory) to determine whether or not specified strings are included.