نٓۚ وَٱلۡقَلَمِ وَمَا يَسۡطُرُونَ
مَآ أَنتَ بِنِعۡمَةِ رَبِّكَ بِمَجۡنُونٍ۬
“By the inkstand and by the pen and by that which they write.
You [Muhammad] are not, by the grace of your Lord, mentally ill.” (Qur’ān, 68:2-3)
Of the various criticisms against the Prophet Muhammad (sa) by modern atheists, one that is especially naive is the notion that the Prophet Muhammad (sa) was somehow mentally ill, and that the revelations he purportedly received from a Divine source were in fact the product of his own disturbed mind. Notwithstanding the amount of prophecies in the Qur’ān that were fulfilled not only in the Prophet’s own lifetime but in the lives of his successors (the Caliphs) as well as even in modern times, this article will instead analyze the possibility of mental illness as the source of the revelations from a clinical psychological perspective. The author, though not a professional in mental health, is studying neuroscience and mental health at the university level and intends to pursue the clinical neurology field in the future, God Willing.
Inasmuch as hallucinations are concerned whether auditory or visual, these episodes of psychosis fall under mainly four disorders, two psychiatric and two neurological, that of schizophrenia, delirium, dementia, and epilepsy. We shall analyze the diagnostic criteria for each and see if they applied to the Prophet Muhammad (sa), after which a discussion on the nature and probable neurological mechanism of revelation (God speaking to man) will be addressed. This research will be released in parts, with Part I (the current) about schizophrenia, part II regarding delirium, Part III on dementia, Part IV on epilepsy, and Part V on the nature and neurological mechanism of revelation, along with instances of such in the scientific world.
Schizophrenia is a severely disabling mental disorder involving hallucinations and delusions, as well as often paranoia and irrational beliefs. The clinical diagnostic criteria for schizophrenia by the Diagnostic and Statistical Manual of Mental Disorders Vol. 5 (DSM-5) are listed as follows:
A. Two (or more) of the following, each present for a significant portion of time during a 1-month period (or less if successfully treated). At least one of these must be (1), (2), or (3):
3. Disorganized speech (e.g., frequent derailment or incoherence).
4. Grossly disorganized or catatonic behavior.
5. Negative symptoms (i.e., diminished emotional expression or avolition).
B. For a significant portion of the time since the onset of the disturbance, level of functioning in one or more major areas, such as work, interpersonal relations, or self-care, is markedly below the level achieved prior to the onset (or when the onset is in childhood or adolescence, there is failure to achieve expected level of interpersonal, academic, or occupational functioning).
C. Continuous signs of the disturbance persist for at least 6 months. This 6-month period must include at least 1 month of symptoms (or less if successfully treated) that meet Criterion A (i.e., active-phase symptoms) and may include periods of prodromal or residual symptoms. During these prodromal or residual periods, the signs of the disturbance may be manifested by only negative symptoms or by two or more symptoms listed in Criterion A present in an attenuated form (e.g., odd beliefs, unusual perceptual experiences).
D. Schizoaffective disorder and depressive or bipolar disorder with psychotic features have been ruled out because either 1) no major depressive or manic episodes have occurred concurrently with the active-phase symptoms, or 2) if mood episodes have occurred during active-phase symptoms, they have been present for a minority of the total duration of the active and residual periods of the illness.
E. The disturbance is not attributable to the physiological effects of a substance (e.g., a drug of abuse, a medication) or another medical condition.
F. If there is a history of autism spectrum disorder or a communication disorder of childhood onset, the additional diagnosis of schizophrenia is made only if prominent delusions or hallucinations, in addition to the other required symptoms of schizophrenia, are also present for at least 1 month (or less if successfully treated).
Now we will analyze each of these criteria that are relevant in turn (with the exception of C-D since they build on A-B, and the exception of F since the diagnosis is for the Prophet when he was an adult).
A.) 1. Prophet Muhammad (sa) did not have any false delusions as schizophrenics usually have, which are most often of a persecutory nature and are opposed to reality. A schizophrenic may think that the government is out to get him/her and may display ideas of reference (i.e. seeing supposed signs of things in neutral stimuli, such as seeing “proof of a government plot” in a newspaper cutting). The Prophet (sa) did not display any such delusions that would usually be distressing to the individual. On the contrary, he was firm in his conviction as a prophet to the extent that when asked by the Meccans to abandon his call as a prophet, he said:
“Even if they placed the sun in my right hand and the moon in my left, to force me to renounce my work, verily I would not desist thereform until Allah made manifest His cause, or I perished in the attempt.”
It could however be argued that he may have suffered from delusions of grandeur (which usually manifest themselves in the schizophrenic thinking that God has chosen him/her for for some special mission or that he/she holds some place of status), however this is also debunked by the fact that the Meccans offered the Prophet (sa) riches and status if he stopped his prophetic mission yet he refused. In addition, he was known as al-Amīn (the trustworthy) and al-Sādiq (the truthful) among the Arabs prior to his mission. In fact, when he stood on a mountain and asked the Meccans if they would believe him if he hypothetically said that an army was approaching the city of Mecca, they unanimously agreed that they would. An entire society of people including its most intellectual would not accept the ramblings of a delusional person. And yet all the Meccans unanimously agreed he was truthful, and later the chieftains of Arab society in Mecca became his followers including ‘Umar bin al-Khattāb, Abū Sufyān bin Harb, ‘Uthmān bin al-‘Affān, and others.
2. It could be argued that his revelations constituted hallucinations, but this will be discussed in due forth.
3. It is recorded in the earliest biographical accounts that when the Prophet (sa) would speak, he would be coherent, speak in a moderate tone, and often repeat himself at least once to make the message clear. This is not the disorganized speech shown by schizophrenics.
4. The Prophet (sa) did not display catatonia (immobility in strange bodily positions over extensive periods of time), which is a (rare) symptom displayed by some schizophrenics.
5. The Prophet (sa) did not display any of the negative symptoms for schizophrenia, which include flat affect (facial expression of emotion) and avolition (lack of motivation to do anything). On the contrary, a companion named ‘Abdullah bin Hārith narrates that “I did not see anyone who smiled more than the Messenger of Allāh.”  It is well recorded as well, that the Prophet (sa) would often cry to let out his emotions, during prayer or otherwise.
B.) The revelations experienced by the Prophet (sa) did not impair him in any way. On the contrary, he was able to effectively function in marital and family life, as well as function as the head of an entire state in Madinah. In addition to this, it should be noted that a very prominent symptom of schizophrenia is the failure to maintain regular standards of self-hygiene. The Prophet Muhammad (sa) on the other hand was known to be extremely particular in matters of hygiene, to the point that he said he brushed his teeth almost before every time he prayed during the five daily prayers. He in fact stated that “Cleanliness is half of faith.” 
E.) The Prophet (sa) did not take any form of drug or intoxicant for recreational use in his entire life, although it was the custom of the Arabs of the time.
In addition to all this, it should be noted that epidemiological studies have revealed that schizophrenia has an onset of late adolescence and early adulthood (especially for males), while the Prophet (sa) received his first verbal revelation at age 40. According to this diagnosis, the Prophet (sa) was not in any way or form suffering from any type of schizophrenia.
1 Sīrah Ibn Hishām of ‘Abdul-Mālik bin Hishām (died 213 A.H.)
2 Tārīkh al-Tabarī (History of Tabarī) of Abū Ja‘far Muḥammad bin al-Jarīr al-Ṭabarī, Vol. II, pg. 228; Tārīkh al-Khamīs of Hussayn bin Muhammad bin Hasan Dayār Bakrī, Part I, p. 279
3 Shamā’il al-Tirmidhī of ‘Abu Abū ‘Īsa Muhammad al-Tirmidhī (died 279 A.H.), Chapter on the Speech of the Prophet (sa), pg. 107
4 Shamā’il al-Tirmidhī of ‘Abu Abū ‘Īsa Muhammad al-Tirmidhī (died 279 A.H.), Chapter on the Laughter of the Prophet (sa), pg. 109
5 Sahīh Muslim, Book of Purification (2), Hadith #1
6 Erick Messias, Chuan-Yu Chen, and William W. Eaton. “Epidemiology of Schizophrenia: Review of Findings and Myths.” Psychiatric Clinics of North America. 30, 3 (September 2007): 323–338.
7 Mirza Ghulam Ahmad. Philosophy of the Teachings of Islam, pg. 13E.