These impairments tend to be present even before the beginning of symptoms such as odd beliefs (e.g., paranoid thinking) or hallucinations (e.g., hearing voices). They can often present as problems with concentration, memory, or the speed of thinking. However, not all individuals with schizophrenia experience these difficulties in a the pre-illness phase. For some individuals, the beginning of the symptoms (e.g., odd beliefs or hallucinations) can occur suddenly and without any prior difficulties, such as problems with cognition (thinking skills).